Isnt the rear of the Pro 4x a Dana 44, and again only if its a manual transmission? If it is a carrier issue, wouldnt throwing an ARB inside solve that problem?
So all Pro4x/Offroad versions (manual and automatic) come with the M226 rear axle (which I have heard is version of the Dana 44). Also the the lower models with the V6 and manual transmission come with the M226 (no locker). Lower model automatics however come with the C200. All Xterras come with the R180A IFS which "seems" to be the real weak point, but as mentioned can be swapped for the Titan/Armada M205 IFS. But I would wager that the R180A and C200 are still plenty strong given prudent use of the skinny pedal.
Essentially, if you are replacing the suspension, the Pro4x only gives you skid plates, selectable locker and a stronger axle but only if you were planning on buying a automatic. If you get a manual the only difference is the skid plates and locker.