if you remove the beeding, you will find the bolts that hold the bumper in place, what you can try doing is loosen the bolt near where the bumper is sagging and push the bumper back into place and while holding it in place start to tighten the bolt again....This is what I did on my previous E36.
This is the front bumper I am talking about, I havent tried on the rear bumper but I would imagine its exactly the same.
Good luck!