I have a 1991 Camaro RS, I bought it for a project and it is drivable, but when I am driving it at about 2200 rpms the rear end starts shaking pretty harshly, but when I push the gas in more it stops. What I want to figure out is why it is shaking and what needs to be replaced so it doesnt shake anymore.

Also the back tires are not brand new and just recently were replaced with used tires same with the front tires, not sure if that has anything to do with it.

Currently what needs to be replaced;
Front and rear shocks
Fuel Pump and filter
Floor boards