xj gears 2wd 8.25 31s and boat
 

I just got a 2000 XJ 4.0l auto 2WD with a 8.25. I assume its geared at 3.55, i have new 235s on it but i want to lift it 3.5-4.5 and get 30s or 31s. ive heard the 3.55 would do "okay" but now i want to tow a 2300 lb boat and an trailer so id be around 3500 pounds for about 60 miles (i live in florida so not many hills).. so would 4.10s help? also i want to lock the rear, i really dont know much about axles but should i do it at the same time?

Short answer: Yes, and a transmission cooler.

Long answer: Comes up at least once a week. Go to Google and add "site:cherokeeforum.com" with your search.

wouldnt my XJ already have a tranny cooler??

Not one worth mentioning. It's a couple of lines in the XJ's already undersized radiator.