info, 38 mph vibe rear diff?? vibration

[melmount]

Another chapter to this 38 mph vibration. I need some info. I found a bad uvch harness and fixed it. The motor really seems to be running good.
I jacked the rear of the truck up tonite and ran it to 38mph. It starts to vibrate as it oes when i am driving it. YOu can see the rear end start to move up and down. I tried 2 different sets of back wheels to eliminate a bent rim.
Weird thing is that barely giving it more gas to go to 39 mph it seems as if it unsticks or something and jumps to 43 mph. Like i am letting go of the brake.
The brakes are not hanging up.
It weird how it raises speed more than what i am pressing the pedal to do. It jumps up 5mph.
If the reardiff starts to shake the whole back of the truck, its not just with the same wheels, does that mean the issue is in the diff?
thx

[melmount]

bump.......

[forddieseldoctor]

Have you checked your u-joints yet? It sounds like you have one going tight.

[melmount]

just got the driveshaft back from a balancing place today. Scuffy828 sent me there. I replaced all 3 of them plus carrier bearing myself 2 months ago. They just went thru it. I have buzz tested, cct test, changed a torque converter, even found the loose uvch. All ohms out well now in the wiring.
The truck shakes at38 when being driven, it bounces in the rear up and down when jacked up, even with no load on it. I do get a clunk when going reverse to drive and another when going back to reverse. It clunks too most of the time whn i come to a stop. I thought driveshaft too.

[melmount]

bump......... could really use some imput. someone mentioned the tranny to me. I dont see how? right at 38 the rear end starts to bounce.

[melmount]

anyone??....

[03_Turbo7.3]

sounds like the gears in the diff, have you checked the bearings? or even the backlash on the gears?