This thread should help you out with the gears:
http://www.vehicross.info/forums/sho...t=gears&page=3
I decided that 4.77 gears are really what we need, but they are incredibly hard to find. After much searching there is still a little debate in my mind after calling around to a bunch of junkyards and such, but basically what you are looking for is:
1988-1991 Trooper with the "Big Tire Package" (which were 31"x10.5" tires and 15"x7" Honeycomb aluminum rims)
Now as far as the option code that should be stamped on the option ID plate in the engine compartment I have found two different opinions for this "Big Tire Package" the two option codes I have been told to look for are:
SL2 or S2J
Also I have heard that the 1989-90 (short wheel base) Isuzu Bighorn imported as a Trooper with a 2.6L 4cyl. had the 4.77 gears, but I am a little unsure of this and have yet to confirm this with many other sources.
As far as 5.38 go you would absolutely need to have the 35" tires and http://www.independent4x.com/ would be able to help you with all the gear stuff. The thread I listed above has all of the rpm and various gearing options with mph and everything, let me know if you don't find something there.
Honestly though I would go for the 33"s if that is what you want and wait for something to break, because a lot of people have warned me and others about the 33"s tearing up so many things with the stock gearing but outside of a few people really pushing the limits of the CV's off road I haven't heard of too many CV joints breaking. Just a though.