Just to start with some easy diagnostics, take any pressure off the driveshaft and see if the pinion yoke is loose.
When they are loose it will do what you described because of one direction causing a mis alignment of the pinion and ring gear.
Also, you said the wheels turn easy.
An open type rear end will let you turn one tire one way and the other tire will turn the other way at the same time.
Put one wheel on the ground or block it and see what turning the one in the air does then.
If the sprag is working at all, one way will be harder than the other.
