Halfway thru unbolting the rear driveshaft on my sons 96 A4 Quattro, and the two bolts that hold the center carrier bearing are just spinning but not coming out. I can see they are mounted in some kind of a sliding nut that runs in a track behind a shield plate welded to the floor of the car. The plate it closed on each ent so theres no way to get into the track from either end. The nut itself does not appear to be spinning in its track, so I'm assuming the threads are stripped on either the bolt of the nut, just don't understand why I can't get it to pull out of the nut while spinning it. Are there any tricks to freeing these up, before i get out the cutting wheel and cut a door into that sheild plate to get into the slding mount and whack off the heads of the bolts?