1999 2500 Suburban, 454.
Can’t get rid of this annoying vibration! At 55mph or more, constant vibration. Under 50mph, smooth as can be. Truck has 80k on it now, vibration has been in it since I bought at 8,500 miles in 2000.

Here’s a run down of what I’ve done so far:
Multiple balances
New tires (two sets)
Had wheels checked for run-out, checked good
Alignment
New shocks
Rebuilt and balanced drive shaft (was not found out of balance)
New brakes all the way around including rotors AND drums
Full tune-ups (like four times)
Scanned with engine analyzer, no faults found.
New motor mounts
New tranny mount
New harmonic balancer
New flywheel

Vibration is coming from the back, or at least it’s manifesting itself in the rear. On car lift in gear at speed, axle bounces. Considered that maybe bearings or gears are bad in rear axle, but that would seem to be evident at all speeds. My only guess left is the torque converter, but I’m sick of throwing cash at this thing to no avail.

Any guesses/suggestions???

u-joints?

Done. Part of driveshaft rebuild. No change.

was the truck a wreck repair?

if it's not straight it'll rear it's head at higher vehicle speeds

and yes it would hop on the lift if the axle (frame) is side shifted.

been there since you bought it used with ...low miles...

there isn't much left to consider Dave.

Don't know if it was whacked, doesn't show any signs of that. Alignment should have picked that up I would think.

cosmetic signs? you'd never know. Alignment? true 4 wheel?

was it off alot and they compensated as much as they could?

was it ANY better, or different after an alignment? I doubt it.

something's wrong in the back of the truck/driveline

just runnin' stuff by 'ya...

have somebody reputable put it on a frame rack and check it to rule that out.

I'd do that next.

if not before throwing all that money at other parts

Have the axles been checked for run out (with a micrometer, not eyeballs)?? Also lateral play while installed.

2 or 4wd??

4 wheel drive.

Haven't pulled the axles and checked for run-out, I don't have any v-blocks. It's a good suggestion and I've considered it. Pretty big job (not compared to everything else I've done) but, I need some blocks. Suppose I could make some.

I said "constant vibration" in my original post, I should qualify that; it's constant when it's doing it. Occasionally, it will fade to almost nil, not gone totally but almost. it's very infrequent that it fades and it will do that while driving at constant speed.
Very strange, almost like something finds an equilibrium or center point for few minutes then goes right back to vibrating it's ass of again. That's why I'm thinking torque converter. Does that sound reasonable?