The factory gauge is not really all that accurate. Before you do anything, put a mechanical gauge on it and see what's actually happening. However, you will probably find that the oil pressure is lower overall than most "specs" say it should be, depending on the age and mileage of the motor and how well it's been treated by you and previous owners.
Is there something else that causes you concern about the motor, or does it run fine as is? If so, keep driving it until it blows up and replace it when it does (it's probably going to run fine for a long time as-is). 4.0's are stout motors, it will run for a long time with lower than "perfect" oil pressure, and if it does go... there are lots of them in wrecking yards to replace it with.
As for potential fixes if it really bothers you that much... the two possibilities that are most likely to be the issue... worn main and rod bearings (very likely) or a worn oil pump (less likely). You could replace either, but you'd probably be better off with a whole engine overhaul to "do it right" when you start looking at replacing rod or main bearings.
Short answer: it depends. What year and how many miles?