car is 2000 z28

Noticed problem when i took car on highway last week. At 55-70mph, the car shakes and doesn't feel safe over 65mph.

When I am going 5mph over cracks in road, the front goes over fine, and the back kinda slams harder.

When I turn to the right the steering is harder to turn that way, and at higher speeds it attempts to turn out of the turn, pull the other way. Turning left is not as hard to do.

I took it to a shop and the said it was rear end, but they also said it was low on POSI additive and the clutch pack was going and wanted to charge me $2000 to fix. SO F*&k them.

I am going to look at this Saturday and was wondering if anyone may know what is up
thanks