Vibration in Seat and Pedals

[Guggenheim99]

Do not know if this is the correct forums for this post. So I have a vibration in my seat/pedals from about 65-70'ish. I've done the following to try and fix it with none of it working:
Had someone balance them, still vibration. Won't matter in the next test I did
Jacked up the rear and drove at the speeds above without tires, still vibration
Took rear drums off and drove at the speeds above, still vibration
Took the diff apart, no issues found with the bearings in there
New U-Joints in the rear driveshaft, still vibration
Got a junkyard driveshaft, still vibration
Checked the U-Bolts on the axle, all are tight
At a loss for what could be causing this vibration. I did take the whole rear driveshaft off completely and spun it up to those speeds, and no vibration at all. That should rule out engine or trans mounts being an issue. Any suggestions or ideas?

 

[adsm08]

Bent axle shaft maybe.

 

[19Walt93]

When did the vibration start, does it change only with speed or is it affected by throttle pressure, is it primarily something you feel or do you hear the vibration as well, is the truck modified/lifted/lowered? If you feel it in the seat and pedals it's almost definitely from the rear.

 

[Josh B]

Had to do my axle shafts last year. In one the bearing, after 340,000 miles, had worn a groove into the shaft. Wasn't aware of the pick-n-pay place then, paid almost 200 for a replacement at O'really's

 

[Guggenheim99]

When did the vibration start, does it change only with speed or is it affected by throttle pressure, is it primarily something you feel or do you hear the vibration as well, is the truck modified/lifted/lowered? If you feel it in the seat and pedals it's almost definitely from the rear.

It started about a year ago. I don't drive at those speeds alot, so I didn't try and fix it as soon as it happened. No affected by throttle pressure. Happens from 65-75 regardless if I am coasting, cruising, or WOT. I can definitely feel it in my seat and pedals. Truck is stock.

 

[19Walt93]

If it vibrates while on jack stands you can have someone hold it at a problem speed and CAREFULLY feel around to narrow down the search- touch the pinion end of the rear end and the extension housing to see if it's in the front or rear of the drivetrain. If it developed on it;s own I'd lean toward something like a u joint freezing up or wearing out(but you said you changed those) or a balance weight coming off the driveshaft. If you ever bottomed it out it's worth checking driveshaft runout front, center, and rear, and reindexing the companion flange to the pinion flange. Using a junk yard shaft may not prove anything for you. Another thought- check the transmission mount, if it's broken a normal amount of vibration can get exaggerated.

 

[4x4junkie]

I would be looking for a damaged pinion yoke/flange at the axle (and also at the t-case flange if 4WD... if 2WD, maybe a bent transmission output shaft). Given the results of your process of elimination, these seem like the only possibilities left.

(a bent axle shaft I don't think has enough mass to cause a vibration if there's no wheel & tire attached to it)

Since this seems most likely a driveline or transmission issue, I'll move your post to there. If it ends up being an axle issue, then it can go there (certainly it is not a steering issue though)