Ok, so I'm still pretty new to TW but for the past few weeks I've looked through just about all threads having to do with any vibration and am still stumped with the vibes I'm getting.
My truck is a stock 2003 TRD Xtra Cab 3.4 4WD with 89,000mi and a replaced frame. To my knowledge the truck was never lifted, however it does seem to sit an inch or so higher than other stock tacomas, which I've heard can be a result of the frame replacement ( http://www.customtacos.com/forum/showthread.php?t=143581
Anyway, the vibrations (like rumble strips, though not as bad) start around 20mph and can be felt all the way up to 45mph (significantly less distinguishable) before completely tapering off before 50. It is an intermittent vibration felt through the floorboards and the pedals, but NOT through the steering wheel. The vibration hits a sweet spot right around 30 mph where it is most pronounced, and at this peak it can be heard as a low pitch "humming" noise (still intermittent). Vibrations persist whether accelerating, decelerating, or coasting while in gear or in neutral. Turning the wheel or swerving from one side of the road to the other has no effect on the vibration.
Thinking it was a driveline vibration, I had my mechanic friend take a look at the u-joints and carrier bearing. After taking the driveshaft off the carrier bearing seemed to be the culprit (as it showed signs of wear and "oscillated" or "wobbled" when spun). After replacing the carrier bearing and one of the rear u-joints, the problem was not resolved or even affected. Now I am beginning to think the vibration is coming from somewhere in the front end.
After getting a good snowfall I was able to drive some smooth snow covered roads and noticed that the vibration seemed to stop when 4WD was engaged. I've read about this possibly meaning a problem in the front diff, but then again that seems to be something that occurs mostly in the 2nd gen's or with tacos that have been lifted, not in stock 1st gens.
I've been driving with this vibration for about one month now and the problem has only become slightly worse. During the first week or so I even drove it home on a 185 mile trip from Buffalo to Cleveland without any noticeable difference on arrival. I'm planning on driving back to buffalo in another week and I'm bashing my head trying to figure this problem out, as I don't feel like driving it long distances before these vibes are resolved. If anyone can think of something that I may be missing I'd greatly appreciate responses. Thanks.