Go to this page and you can play around with gear ratios and tire sizes and it will tell you the RPM you will get.

http://www.csgnetwork.com/multirpmcalc.html

As another thought for you to get by on the cheap for a while, maybe you should pick up another set of wheels and tires for your highway driving. Running a set of 31's would probably work out pretty well for you on the highway until you have the cash to change your gears. Might look kind of funny with a 5 inch lift and guppy tires, but it is worth a thought.

haha    i have thought of it tho i would like the lower gearing. ive also thought about looking for junk yard cases. im putting thought into it quite a bit. although im also looking around the boards to see if i can find a set of 5.29 equipped thirds so i can just swap out. i usually see em on here once in a while. some even come with lockers in em so id figure it couldnt hurt to search.