First thought...I'd agree with Kliff, sounds like the brake is too tight.

If one diff is installed backwards the truck will not roll at all with all four tires on the ground. However, it would roll freely with either the front tires up or the back tires up.

Lastly, the bulkless set up I had (FLM) you had to make sure the bearings were seated properly or the diffs would bind when the case was bolted together. A slight tap on each bearing to seat it cured the problem.