I have absolutely no knowledge about cars other than Hondas, but here is what sounds logical to me.
If your Bronco usually uses only rear wheels to move, and it won't move by using rear wheels only, but wants to move using 4x4 (means all wheel drive) then perhaps something that brings the power to the rear wheels is broken. Try jacking the car up on jacks, and turn on 4x4 and put it in drive and see if all wheels turn.
If all this makes no sense - ignore me
