Bummer that you can't adjust them looser without taking off the wheels. Hopefully you will get a good "feel", how tight is not too tight, but rather just right.
You do understand that in order to back off the brake shoes, you have to push in the lever with another screw driver to unlock the teethed wheel, right?
You need to work with 2 screw drivers inside that little slot, one to push in the lever a little, the other to turn the teethed wheel in the loosening rotation. As soon as you release the lever, it pushes right back against the teethed wheel and keeps it from turning in the loosening direction.
ADDING: Just a reminder that when quickly backing off on the tightness, your old 1993 E350 parking brake should NOT be set for it might make it extremely difficult to back it off. I believe the older E450s have the parking brake located on the drive shaft so that suggestion would not apply.
