Odd vibration

Sorry this is long, but I wanted to give all info ...
98 SR5 4X4, V6, ext. cab, auto trannie - I bought this truck from my daughter who owned it for almost 10 years. (She got another car.) It has 240K miles, has had regular maintenance, and has always run perfectly. Love almost everything about it with one notable exception - it has this weird vibration/harmonic at almost all speeds above 10mph or so. On perfectly smooth road, it's minimal. But if you hit a rough patch, the only way to describe it is that the suspension turns to jello. Lots of shaking and shivering, and like there's this weird harmonic going back and forth through the whole truck. (Don't know any other way to describe it.) There is no squeeking/growling or other noise associated with this, and it seems to be coming from the entire front end, not one side or the other.
The truck is completely stock, including ride height, except for aftermarket wheels and tires - American Racing aluminum wheels and 31/10.50/15 Bridgestone Revos. I'm pretty sure it might have newish shocks, but her ex-boyfriend put them on and I've been instructed that we don't talk to him anymore so I can't ask. It had an alignment 3-4 months ago. I just took it back to the tire place which rebalanced and rotated the tires - no difference in ride quality. I have experimented a little with different tire pressures - and the problem seems to be slightly lessened running them at 35ish pounds - which I've read may be a bit low. I have jacked the front end up and wiggled the tires side to side and up and down - no discernable play. I've grabbed and shook and twisted everything I can get my hands on down there and, again, no play or loose stuff I could find. The truck also has new brakes and rotors because it had shakey brakes when she first got it and she thought it was getting them again. It's not. The brakes stop it straight and true.
The vibration does not affect steering (steers fine even on max vibration). Turning makes no difference, better or worse. As mentioned, the brakes themselves do not vibrate.
I've always worked on my own cars and have had many of them, but this issue totally baffles me. Might anyone have any suggestions or experience with a similar problem? Only things I can think of are that that the combination of big tires, agressive tread and the permanently locked front hubs (and always-spinning front diffy) might be combining to produce the problem.
Oh, and this all happens in 2WD. This truck has never been off-roaded and the only time we've ever used 4WD is in ice or slippery conditions.
Might sound strange, but it's so irritating on this otherwise nice truck that I will sell if I can't fix it.

 

drive line issues (ujoints, carrier bearing), wheel bearings, tranny mounts (unlikely), and front axles are what come to mind. maybe a bad clutch but that's very unlikely. aggressive treads might just be the problem. the locked front hubs could be problem. why are they permanently locked? my 00 taco has permanently locked hubs, but that's by design for auto locking hubs (which was i think the first time it was introduced) but it doesn't have any vibration issues.

where do you feel the vibrations? in the steering wheel while driving?

 

Possibly front diff driver side bearing. Common problem with the Toyota 7.5" stock clamshell.

 

Thanks for the replies. In answer to "why are the front hubs permanently locked" - sorry, I'm not that well versed in 4X4, but this truck does not have manual hubs. I know the selector lever inside engages the transfer case and front driveshaft. But since there seems to be no provision/switch/lever for engage-disengage of the hubs, I am assuming (and have read) that they are permanently engaged, which means the front differential axles and gears are turning all the time. Is this not correct? I know when I've driven other people's 4X4s and someone -okay, me - has forgotten to disengage the manual front hubs, that you can definitely feel it when the truck is on the road as a sort of vibration or harmonic. It's similar to what I'm experiencing with the Tacoma. Just his extra "hum" and shimmy that shouldn't be there, you know?
As for where I feel the vibrations, the best I can do it "in the whole truck." The whole dang thing does it, but me and everybody who's felt it agrees it seems to start/originate in the front end, and it's worst in the front. I would say with some confidence that it's not in the steering - when it happens, there is no change in steering, which is nice and solid. If you momentarily let go of the steering wheel, you can see the wheel shake and shimmy, along with everything else in the truck. But grip it again and its nice and solid.
Visage58 - I also suspect that aggressive tread on the Bridgestones. I'm thinking the next plan of action will be to swap wheels with a buddy whose same-vintage Mazda 4X4 for some strange reason is fitted with Toyota rims and stock-size 265 tires. It will probably cost me a sixpack, but will at least confirm or eliminate my present tires as the problem.
TomTom - By "front diff driver side bearing" do you mean the wheel bearing? Or the bearing inside the pumpkin? I've read on the forum here about bearing issues, and aside from the vibration (which seems to be coming from the whole front end rather than one side - but I could be wrong), I don't hear any noise or feel any "play" when I jack the front up and shake the wheel. But of course if the bearing is inside the diffy, maybe I wouldn't. If that's a common prob, I'll read more about it and see how to diagnose it.
If I keep this truck, I'd like to convert it to manual hubs which seem to me to make much more sense for my mostly street driving - why have all those axles and gears and bearings spinning up there when you're not using them?
Thanks again for your input.

 

oh you have auto locking hubs. then it's definitely not form the hubs. check other locations mentioned

 

http://eastcoastgearsupply.com/i-14472912-toyota-7-5-clamshell-bushing.html

 

UPdate: After re-aligning, re-balancing and rotation, vibes were still there. Once again, got it up on the rack everything in the front looked pretty tight. I gave everything I could reach the grab-and-shake test. Was aghast to find a broken shock in the back! The body had snapped off the bottom bracket but was lying against it so you didn't really notice until you gave it a shake. New pair did wonders for the truck. About 60% better, but still vibing a bit. So I fixed the other thing I found: a wobbly spare tire. Yeah, crazy, right? But the way the spare was hanging, it's just supported by that one bracket across the bottom of the bd and the tire flopped fore and aft quite a bit. I quick-fabbed some wood pieces to support the tire better and - again - some of the vibes went away. I'd estimate that about 75% of the vibrations are now tamed. Not all gone, but certainly tolerable. About 1/2" of play in center bearing, so will tackle that next. But getting there!