After you are sure the rear end is thawed out, try pulling the shift lever out by taking the 2 bolts out at the shifter base.  Sometimes as the shift linkage gets worn, the ball on the end of the shift lever will jump out of place in the 2 shifter slides that it rides in, causing it to not shift between ranges, but still shift into the gears in which ever range it is locked into.  Just make sure the notches in the shift slides are lined up evenly, and drop the shifter back in.  I have done this several times, and so far I have had good luck that this is all that was wrong.  Hope this helps, Terry.