rear sitting lower than front

I have a 88 gt convertible 5.0 auto what causes the rear to sag and what is best fix open to suggestions

Sounds like it needs new shock absorbers. They did equip some 5.0 with adjustable shocks. You will have to look at them to be sure. If they are, your pump to them may be bad as well as the shocks.

Shocks would not affect right height. The springs are what would need replacing if it is sagging. You;d probably want to replace both at the same time anyway if you are replacing the springs.