I had a problem once where I lost the set screw out of the "Center Joint Cup" (part #FF047). Since I only lost the one out of the front, the back wheels would still turn, but the front wheels stopped pulling. If you look at the shaft that comes out of the transmission and goes into the differential, there is a small set screw that holds the cup onto the differential. These are very cheap if that's the problem. It is worth a quick look and you don't have to tear anything apart to determine if that's the problem.