tires move in and out?

Ok so I’m chasing down a vibration above 65 MPR. Got the car up on jack stands and found that my back tires move in and out. Does anyone know what makes this happen? It’s a 92 GT with 5 lug conversion, 3.73 gears, aluminum driveshaft, tremec 350. Any help is appreciated
 
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You will see some in and out movement. Thats just the slack between the C-clip and pinion shaft. But too much could be a problem. In and out shouldn't cause vibrations, but up and down will.

Could be the drive shaft. Wheel out of balance, or out of round. Out of alignment can cause this too.
 
Mine doesn't move. Only movement i get when i rock the car side to side is the tire flexing on the sidewalls.

A while ago i did have movement and it ended up being my control arms not being torqued to spec.

I'd suggest going under the rear of the car and have a friend push the car side to side. See if you can see where the movement is coming from usning the upper arms and tailpipes as reference. Does the axle move in relation to the body? or does the movement come from the brakes/axles in the housing
 
His car is on jackstands. And his is pulling the wheel in and out. He is not going to witness tire flex while on jackstands. Nor control arm bushing flex. Thats assuming he has the car supported by the axle, and not the frame.
 
if it's just one side it's probably a worn out c-clip, you'll want to replace it very soon or watch your rear wheel pass you. If it's both sides it could be a worn pin. Just plan on servicing your rear end and pull the cover and see what's up.
 
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This shows the pin and the c-clips. The pin is actually called the gear pinion shaft. The lock screw is in a little different place than what's pictured. You should be able to see if it's worn without removing anything other than the cover. You need to remove the pinion shaft to see the c-clips, once the pin is removed you can push the axles inward to remove/replace the c-clips. Just don't let the spider gears fall out once you pull the pinion shaft or axles, it's a pitb to get them back in.
 
Well I jacked the back of the car up again and started the car. When I put it in gear I can see the tire wobble in and out about 1/8 to ¼ of an inch, so I took the tire of and the axel flange is moving is moving bent I guess. Both sides are doing the same thing. Is it common to have a bent axel? O and I took my rims and had them dynamically balanced- checked good.