I'm assuming that you have the solid rear axle and not the independent rear with nivomat shocks.

I would either drive from the passenger side seat(which is tough to do) to even the car out or just get new springs for the rear. They're pretty easy to swap and aren't that expensive to replace.
Everybody including myself love IPD's lowering springs as long as you don't mind a firmer ride. You will need a HEAVY-DUTY spring compressor to change the front springs. Our Volvos use larger than usual springs in the front compared to most cars and regular spring compressors just aren't safe to work with. If they let go you will be in a world of hurt.