I drove the CJ5 (manual tranny)in the sand and mud with very deep rutts. 4Lo forces me to go slow enough that I do not bounce over the rough rutts and stuff... Not that I needed the extra gearing but flat out top speed was about 35 MHP. In 4Hi I would find my top speed closing in on 45-50 MPH... That was just a little fast. My best hill climbs were in 4Lo -Second gear.... It just felt like it climbed better in 4Lo second than it did in 4Hi first or second. Could also purly driver preferance.
While not in the sand there were plenty of times where 4Hi first gear was simply too fast. One gas... One Clutch... No Brakes..
In my XJ (DD) I use 4Lo less... I am not sure if it is because the XJ has a slushbox or if I am not pushing it to the ends of its limits like I did the CJ. Anyways I, believe that normally I do not NEED the gears but I want the ability to go slower (less MPH) with the same engine RPM.
For most of my wheeling trips I think the XJ would do just fine in either 4Hi or 4Lo. I use 4Lo just to make sure I am going no faster than what is required. The type of wheeling is also differant... my xj gets more light trails and mud puddles than deep mud, off-camber, crawl over that thing that the CJ did.