Sounds like an axle problem but I assume you checked them before you pulled the tranny. I think it is prob a diff problem. With no load on the tranny you can turn the input shaft and the diff will spin. You need to put the axles back in the tranny and have someone hold them while you turn the input shaft (make sure it is in a gear). This will let you know if it is the diff or not. If you open the tranny you may not see any problems if it is in the diff. Good luck.