You would pull all of your spark plugs and run a compression test on each cylinder, annotating the results on paper. If you have any cylinders with low compression, squirt about a teaspoon of oil in that cylinder and retest the compression. If it is your rings that are worn, you should get a good jump in compression on the wet compression retest. If there is little to no change, you may have bad valves. Check Youtube for the process, you may find it better explained. 120K is not a lot of miles on a 4.0, but parts can go bad. I had a tight engine, no leaks and ran well, but a couple lifters chattered occasionally. I found a ridiculously low mileage engine for $500 that had a very gentle life. I ended up swapping out the engine, tranny and transfer. I couldn't resist. My old engine only had about 90K on the clock, but it was not pampered by the previous owners. The quest for mechanical perfection is part of the fun of the hobby in my opinion.
What is the proper psi for good compression?