Ya, it sounds like something wrong with the gears in the front diff. The only other way I can think of to easily test things out is to disconnect the rear drive shaft, keep the hubs unlocked, and put it in gear in 4wd and see if the CVs spin. Sounds like your hubs are working fine.

 

Okay so I looked at the diagram of the ADD and Differential. At least i understand how that is supposed to work. The way it looks, the Rt CV should always be engaged to the diff and should turn anytime the front propeller shaft turns. Looks like the ADD just ties the Rt and Lt together.