OK here's the deal. 3.0 V6 Ext Cab pickup.

When I hit bumps something is pushing my transmission and transfer and I guess motor forward. I can watch my shifters move forward and back in phase with my rear end compressing and extending. I had a bad UJoint at the diff and replaced that and greased my splines but it is still doing it. The truck also shakes at 40-45mph, don't know if it's related though.

Has anyone else had a problem similar to this??

 

My auto shifter moves back and forth about a 1/2" over bumps.

Never really worried about it. Maybe I should.

 

It's normal for the shifter to move left and right with engine torque, and it does shake some. But what you're describing sounds like maybe you filled the rear driveshaft slip joint full of grease and it can't compress. Try removing the zerk and drive it and see if grease comes out of the zerk hole. You might need to separate the driveshaft and clean it out.

 

I was under the impression that that was impossible as the grease would be forced past the splines on compression. there is a gap there afterall...

 

It was doing it before and after the grease.

 

Not impossible at all, you are not supposed to fill it up. Sure there is a bit of a gap, but it's small. It has to be small or the thing would wobble... The grease won't just come squirting out of there easily if it were overfilled. If you look at the slip joint on your driveshaft you can see how much it moves - mine moves a good 2"!