There was a thread here a while back where someone did the same as you - bought a rebuilt box from a parts house. The box he got was different than the OEM boxes - had four bolts on the top and the piston was not interchangeable with the parts from the two wheel drive box to reverse it.
The photos he posted made it look like the bolt pattern to the frame was the same, but the top portion of the box was completely different than the OEM boxes we use to put together the 4x4x2.
I'll have to see if I can find the thread.