The reason the older diffs can't be retro'ed into the newer TT4 trucks is due to the severe handling problems that the higher bias ratio creates in the TT4/ABS system.
BTW, I just installed a full-time mechanical locker in the front of mine. Elockers are for sissies!
In spite of what you may think, in the heavy-ass Hummer, they actually work pretty well and don't affect street handling much at all, at least in non-icy driving conditions.