On a 4x2, all 3 transmissions (42/44/46RE) are generally interchangeable as long as you use the right mount on the tailshaft and the matching length driveshaft. On a 4x4, I don't know as much about the mating bolt pattern and intermediate shaft spline count. I would like to say that they are interchangable, but you'd need the matching set of driveshafts if that were still the case. I know the 42 and 46 are different lengths. I'm not sure where the 44 falls.

 

The 44RE is in the same transmission case as the 42RE, just an extra friction and steel per clutch pack and dropped behind the 318. The 46RE is longer, so they would need a change of driveshafts.