1994 f150 - began hearing a humming noise from the rear end, then the truck lost power to the wheels, the drive shaft spins but not the wheels, the right rear tire and axle try to exit the housing during attempted tow. what's the problem?

 

The axle that tried to come out is either broken, or the spider gear and c-clip from that axle have come loose. Either situation will cause noise and power loss. If the axle broke, there probably won't be too much damage in the differential. If the c-clip and spider came off, there is probably a small disaster inside the housing. It looks like it's time to buy some "DIFF REBUILD IN A CAN" and spray it in there. Maybe two cans. GlennMc.