Slow 2wd, fine 4wd?
 

99 Blazer, was driving down the driveway and heard a pop and it kind of jerked a little. Now in 2wd it wont go over 5mph and it revs up to 2k rpm (haven't gone higher to risk it) but in 4wd it drives fine. No grinding or anything that I can hear. Not sure what it could be, haven't looked at it yet. Thoughts?

EDIT:
Also, it moves fine in reverse in 2wd and 4wd.