If you can get ahold of a GMC manual (41-46),maybe on-line, there is an explaination on how to adjust the doors. It invloves putting a block of wood at the hinge and closing the door on it, this tweeks the door to move the alignment. It's a little involved, kind of trial and error.
If you can't find anything on-line let me know I may have a copy of those pages I can snail mail you.
George
