W-series is stronger than G-series. R-series is ideal but it's 1/2" longer and requires a special transfercase input gear.
Here's the break-down on Toy 4WD trans:
Weakest L-series
2nd weakest G-series
good W-series
damn good R-series
Kyle is right about your transfercase. You have a chain drive t/case right now. I guess the eaisest way to replace it is to find a G52 or G54 and put your G58 tailhousing and output shaft into the G52/54 trans so you can use your chain drive tcase. Otherwise, if you dont mind upgrading to a gear drive transfercase, then use a 1989-1995 style W56 that uses a forward shifting transfercase that should match the same shifter holes in your cab.
BigMike
