The beefiest axles are found behind the 3.5l trucks. I'm assuming that yours is leaf sprung. If so, I'm not sure if the larger locking 3.5l axle was available in a leaf sprung configuration. Seriously though, you are much better off staying with the Mitsu axle for its strength.

If you want a more economical gearing option that gives you some REAL noticable improvement look here:
My dual transfer case swap.

'06 Dakota QC 4.7 6sp 4x4
'88 Mighty Max
'77 KZ1000 fun old bike