Aus jd-
It sounds as though you are thinking it's loose because of the way it feels, rather than by actually testing it.
To test the diffs in the car, you need to hold the spur gear as well as one rear wheel (or front if you'retesting the front diff) - then try turning the opposite wheel. It should be very hard to turn.
If the diffs are out of the car, then you need to stick a screwdriver or hex driver into the slots on both outdrives, then try turning the pulley.