Loud vibration when lifting off of gas pedal at highway speed

dmage said:

I get this very loud, deep vibration noise in the cabin when lifting off the gas at highway speed. Noise goes away as soon as I hit the gas again... ...I did check the u-joint, there is some play where the U joint hooks into the rear diff. The joint just minimally slides in those retaining brackets that you bolt up the joint using 11mm to the rear diff. I'm not sure how much play is allowed there... could that be it?

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dmage said:

It could also be a dying rear diff (hope not). It was basically dry when I bought the truck but all noises went away completely when I added diff fluid. I can spin the shaft about 1/6th turn without any resistance though, not sure what tolerances are allowed unfortunately.

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Schurkey said:

Yes. There should be ZERO "play". In fact, I'd expect just the tiniest interference fit. A hint more "snug" than a real "slip-fit". The retainer straps may be distorted/worn by now, also.


There's several things that can cause excess driveshaft movement like that.
1. Worn ring/pinion gear
2. Worn differential pinion/side gears.
3. Worn axle/side-gear splines
4. Worn cross-pin
5. Worn bearings (but you'd maybe notice the pinion gear being "loose" or the ring/pinion being noisy.

If the rear axle was run dry, you've maybe got all those things going on--a combination of problems.

1/6 turn is huge. Pull the cover, inspect the "guts" for wear. Keep the wheels motionless, turn the driveshaft--see what moves and what doesn't move.

When my 10-bolt axle got loose like that, I got rid of it in favor of a 14-bolt semi-float. "My" axle got crammed under a friend's truck when his 10-bolt exploded and broke the case and axle housing. I'm surprised "my" axle is still working--but I bet he doesn't put a thousand miles a year on that truck.

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