Your problem description sounds fixable. The overdrive can be removed and rebuilt without a lot of trouble. A pressure gauge test on the bolt sealed port inboard of the electric solenoid would be a good idea.

15 psi running on a lift at speeds over 30 mph and 400+ psi when engaged is what a good relief valve does. 15 psi and something approaching 400 psi and then falling off is waht an overtravelled piston looks like.

The bars can be straightened and the piston seals replaced. The valve for OD is serviceable and is avaliable at a Volvo dealer.