When did this start? You may not have a t/case issue, you may have a broken diff, axle cv joint or spider gears.
Put the t/case into H, start your engine, put the tranny into drive and have someone watch both drive shafts to see if one is spinning.

 

Dont know why I didn't think of it, but check fuse #12 in the fuse box under the steering wheel. It controls the t-case solonoid