Not trying to belittle GoJeep's/Jason's research, but I wouldn't trust just the chart itself. Case in point, my XJ had a 29 tooth speedo gear, stock. It's a 98 Sport with an AX-15 and NP231. Not sure what the stock size was since I got it used (presumably 225/75R15), but the door jamb says 215/75R15. I went up to a 30x9.50 MT/R, and checked my speedo with a GPS unit. Comparing the satelite speed to my dash speedometer, the dash speedo read 2MPH faster than I was actually traveling. This means that my rig should have come with 31s from the factory if the speedo was to be anywhere near correct!
GoJeep's chart said I would need a 28 tooth gear, but a 30 tooth gear is what I needed. Ordered the 30 tooth unit from the stealership, installed, and checked it, and it is 99% accurate with the GPS unit, almost no margin of error.
I would say if you can get a hold of a GPS unit, use that to calculate how much your speedo is off, see what tooth speedo is in your t-case (it takes like 2 mins to check, it's stamped right on the plastic gear), and calculate how many more teeth you need/don't need.