I've been trying to find a way to do it with little luck. I guess it depends on how much money you want to spend. You could find a later eaton rear chunk 4.10 or 3.90 would make a real difference with your current setup and then install the T5 to improve your top speed.
If you want to spend a bunch of money with AA you can adapt a WC T5 to a toyota passenger side drop transfer case (the transfer case is cheap).
How about keeping your SM420 adapting it to the NP205 and putting a ranger gear splitter in front of it (big bucks)?
I am just throwing these ideas out, none of them are cheap.
Do you want 4wd for wheeling or just the look? Are you wanting to keep it 8 lugs?
Chris
