Couldn’t find a forum thread that this fell into, so here it is.

Truck - 1978 F150 S/C long bed 4X4, heavy half (F250 suspension), 460, C6.

I have a vibration, hard, fast (not a shake in the wheel), and seemingly coming from the middle to rear end of the truck.

Facts:
-It appeared after a 4x4 run, was definitely not there since the tires/rims were installed.
-There are scrape marks on a 1.5' section of the driveline, but not around the driveline, they are down the driveline, caused by moving slow over an obstacle (guess).
-All 4 tires (235/315/16" BFG Muds) have been rebalanced. I directed the tire shop to not worry about looks, and put outside weights on when needed.
-Vibration starts at 35MPH and stops at 45MPH
-If accelerating quickly (stomping on it), vibration is almost negligible (but that could be b/c I'm going through range quickly! )
-Vibration occurs when accelerating, AND when decelerating through that speed range.
-Vibration did change, slightly, after rebalancing all tires, but it only shifted the speed range when it occurs, from about 38MPH to 48MPH
-Driveline is a 2 part driveline
-After vibration appeared, replaced the center bearing in driveline, and all u-joints (spicer)
-Rear end is Ford 9"

Any ideas on the cause?

Thanks in advance!

 

Sounds like you bent or dented the driveline causing it to be out of balance.

 

I figured that a bend driveline would cause vibration all the time, not just over a 10 mph range, though....
?

 

how about weights, all there ? maybe got knocked off. also take alook at trans/ transfer case mounts..... just a thought.

-All caps removed by Admin-

 

Vibrations are harmonic in nature and some will not be there at all speeds all the time. Some will just be at certain speeds. It all depends but if it's a minor balance issue it could very well just be at certain speeds.