violent vibration from rear of truck

My truck started vibrating really hard at 45 mph. You can let off of the throttle and it stops vibrating, but as soon as thefuel is reapplied it starts vibrating hard. I jacked up the rear and put it on Jack stands, started it and put it in drive and it starts vibrating at about 25 mph that way as apposed to the 45 mph on the street... My first thought was thrown a wheel weight so I took it to the tire shop...tires were fine...then I put a different set of wheels and tires on it...same thing...next I thought I threw the balancing weight from my drive shaft but it was fine...next it thought carrier bearing ...changed it...still vibrates...next I thought must be one of the universals but I checked all of them and they seemed to be good....so I thought it was my rear bearings...so I went the the hell of changing both bearings and seals....nothing still vibrates... has anyone else had this problem? Truck has 207xxx miles and it's a dbl cab auto

 

all I can think of you haven't looked at yet is the leaf springs, check all the connections and u-bolts.

next I would look at possible transmission or transfer box issue

if all that checks ok then I cant think of anything else and you already went into the rear end so I don't think you would have missed any problems with it while you were in there.

 

Leaf springs and connections are tight

 

The only reason I don't think its the driveline is because you said when you take your foot off the gas even at those speeds the vibes disapear... If it were the driveline it should vibrate at those speeds regardless of gas pedal or not.
If it is driveline related the only thing I could think it would be is severe axle wrap because it only happens when you press on the gas pedal... Pressing on the gas pedal = torque to the rear axle and then by releasing the pedal = less torque but your still going the same speed.

Maybe a cracked leaf spring or really worn out leaf springs...

Edit: jack your truck up again but this time have a friend gas it as your watching what is going on..

 

Check the oil in the rear diff. Why? I dunno but wouldn't hurt.

 

New leafs...well about 6 months old...their good
I checked the axles when I changed the bearings...I put them in a lathe at work and put a dial indicator on them...and I changed the rear end oil as well....could a universal be bad and not have any play what so ever?
Only reason I ask is I spoke with a guy at the stealership and he said his Chevy done the exact same thing when he let off the fuel...but his u joints had play in them he said...when he changed them it fixed his truck....mine have no play at all
universals or rear-end gear chunk is the only option I have left...maybe transmission output

 

Sounds super odd.. I would still at least jack it up and try to actually watch what is going on.

 

Yea I will tomorrow

 

check the torque converter bolts haven't come loose or maybe a cracked flywheel

 

I had the same problem recently, check the u joints. I had one that was seized and was only noticeable when i was on the gas

 

I had a similar problem. When I checked my U joints they seemed OK, but someone one suggested to jack up the rear end and put it in neutral. When I did I found a loose one. After replacing the bad U joint the vibration went away.

 

Ok I lifted the rear and put it in neutral and found my rear u joint has some play...I'll change it out today...hope this fixes it

 

Changed the rear u joint and BAM no more vibs! Thanks guys