I'm running 37's as well. (MT/R's) granted the boggers weight alot more. But, with 5.38's in the axles it does real well. The only upgrades I have done is the 62mm TB reamed the intake to 62mm. And a Rock-it city air intake tube with a K&N. No problems with doing 70-75 MPH. Only on the steepest grades do I have to get down to 3rd gear.
Proper gearing will give you the most bang for the buck as far as peformance goes IMO. But is sounds like your not looking for more get up and go on the street. For rock crawling a second T-case like a Klune or a Altas 4 speed or Mad Rooster box will give you plenty of gear choices for crawling.