I am trying to diagnos a issue with my jeep. I have never had any trans or axle trouble, and now when I put the jeep into gear, I do not get any traction. The drive shaft spins and when the truck is on jacks the tires will spin; but on the ground I can not go forward or back ? When I lock the hubs abd try 4x4 it can not pull itself. Any suggestions, before I tear down the rear-end ?