I have this problem with my 8 3/4. After drag racing at E-town I swapped the 3.91's out and put my 3.23's back in. ever since there has been this thumping sound coming from under the rear of the car. I thought it was just that the fluid needed more sure-grip lube. I removed some fluid and topped it off with more sure grip lube. That did not help. It was also leaking from around the center to housing area. I remembered when I put the center in I had to start one side and tighten two nuts to get the driver side to where I could start all the nuts. so I loosened them all and opened the rear without removing the center, lined it up properly and torqued all the nuts in proper sequence, and refilled the rear with fresh fluid and sure grip-lube. still thumping and still leaking.
Do you think I could have warped the center section to cause the thumping and leaking?
I did use a new gasket when swapping gears but no gasket sealer.
The thumping is rhythmic in nature and increases as speed increases up to about 30 mph where it is no longer apparent. It dosen't shake the car or the steering wheel and I can't hear it when the radio is loud, but I don't like it.
Any one have any ideas or suggestions?]

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My car had a similar sound and it was the worn u joints.