1987 Ford F-250 Rear End Probs

This is a friends truck. Its a 1987 f250 2wd.
He was haulin stuff one day and he said
he heard a loud pop. He drove a little ways further
and he lost all forward movement. He got out and the drivers side wheel was like sticking a foot out from underneath the wheel well. He chocked the wheels, just in case, started the truck and put it in drive. Revved the engine, with no movement. Went back to rev and did same with same results. So he got out of truck with it in drive and looked under the truck. He said the whole driveshaft was spinning so he knows its not the trans. The u-joints are ok. Question is, what in the heck broke in there?
Did the yoke break off inside rear end or sumthin'?
Thanx