Probably the motor stuck from inactivity, but could be the GEM.
Try placing the selector in 4WD high and drive on a bumpy road. Works sometimes; if not, get under and play with it (maybe tap it with a hammer or remove, clean and lube).
If it's the GEM, your screwed ... uh they are expensive.
Remember to engage a couple of times a month (like I never remember to do).