gearing question sorta

quick tech question. The xj were picking up monday has a ax15 4.1 low range and 5.13s on 39.5s. after some calulations it looks like crawl ratio will be 80.56:1 second gear will be 49:1..... high range first gear would be about 20:1 stock low range first gear is about 33:1... vehicle is multi purpose mostly trails, washouts and some roccks and occasional sadly mud..... now i know the HIGHLY reccomended RPM from tera low is 2500-3000 for short periods of time I have a feeling that 1st gear high range is going to be too high and the low range is going to be too low.. when it comes time to mash the gas I'm afraid of burning the gears out letys say if 3rd gear low range in the mud. or on a washout hat wheel spin is needed... I was thinking to myself that if there was a way i could plumb a cooler into the 231 .... if so what would yall think.... for those who know about the tera low 5 planetary setups

5.13s are not low enough for that size tire. 4:1 is cool but your stuck with a drastic range shift. Plenty of guys (including me soon) run 5.13s on 35s. I'll be running it with an ax15 with a doubler 231 and a d300 giving me a crawl ratio of 140. And ill still have the option of two higher ranges. With a stick you should be looking for something closer to 150:1 crawl. If you want to stick with the 4:1 it's time to regear those axles deeper but I think a doubler is in your future. Or atleast a case with deeper gearing.