Some times when i hit 30 40 or 50mph my rear wheels vibrate or shudder for a second. Anyone know why?

 

Could be a number of things. A bad alignment, bad shock(s), bad bushings, bad tie rod... ETC. Lift the front of the car up and see if the tire moves on any axis besides the one its meant for. Just grab it and make it move it any direction but around.

 

Shocks or bushings is my guess, based on the age of the car.

jdaniel, he said 'rear' tires.

 

Whoops, I was thinking front tires because me and a friend was troubleshooting his today. If the rear ones vibrate or shudder then the worst case scenerio would be a bad axle. But check the shocks and the the bushings in the CA's. There is a lot back there that can cause a shutter or vibration also.

 

These are all great thoughts...

Get them balanced. If it's worst between 50-60 mph. Thats the problem.