I have a 700r4 with a decent shift kit. About 2 weeks ago, i started hearing a thud when i shifted(like something is moving around), and if I am reving pretty high (4000+) and i shift, if feels like the rear shifts left and right (pulling either way as the posi changes which wheel is spinning).

I thought it was the tranny mount, so i changed to a poly one, but the old one looked fine, I also checked the torque arm bushings at both ends and the panhardbar, all of which look fine.

So basically i cant find anything damaged or out of the norm on any of the rear end or drivetrain parts... so again I am lost as to what is causing this problem.

The only thing i can think of is that both my engine mounts are pretty much just the metal casings now, the rubber is almost totally gone. Could that be the cause of this?

 

when i got my TA, their was a spring missing (poor mans shift kit) and the wheels spun with every shift, maybe you broke a spring in you tranny, try the tranny board, I know jack about auto trannys