I gave it a basic tune-up to include: Plugs, wires, filters, cap, rotor. Replaced the O2 sensor, fuel filter and the fuel pressure regulator. None of this has helped. It still will not idle unless my foot is on the gas. Any input would be great as I feel like I am stumped at this point.
does the cel come on when you turn the key on before cranking it?
if it does then turn the key on-off 5 times really quick then leave in on position for couple seconds it will start blinking. you will get several sets of blink sequences. example. blink blink pause blink =21 blink blink blink pause blink blink blink = 33 etc. Then compare those to the codes to see what errors you have. you can use the search feature to find the thread for the codes or look in your Haynes repair book.
No CEL light and its running rough could be a few different things. It sounds like you've turned a wrench so start with the vacuum leak check. Take a can of starter fluid or carb cleaner and spray around the engine looking for a change of idle for a vacuum leak. While you are at it, clean the throttle body really good too. Is it any different at idle cold and warm? The next step would be the temp sensor. There are 2 of them; one for the guage and one for the Engine controller.You want to check/replace the one for the engine controller. It is in the front of the block by the thermostat housing. Check/replace the TPS sensor. BTW the idle pressure should be 31PSI on the fuel pump. With a new regulator I dont know how it is reading high with the vacuum hose on and off. The fuel pressure should increase as vacuum increases.
To check the TPS, you need a multimeter. The center wire on the harness is the TPS output. If you backprobe this with the key on, these are the readings you should get - idle more than 200 milllivolts, WOT less than 4.8 volts.
If you actuate the throttle plate, the TPS should follow smoothly.
It may also help to pull your idle air control motor next to the TPS and clean the pintle with a soft cloth and some alcohol, and check the bore for blockage.
Might want to do the fluid change also before you give it to him, esp if he is going to take it offroad. 9 times out of 10 this has not been done in a while. I just did mine this weekend on a 95' I purchased as well.
You need to (I am assuming it's a 2.5L 5 speed manual):
1.) Transmission fluid change (very easy, 15/16 socket head, takes 80w90 gear oil - use synthetic, fill until it's full - almost dripping out the fill plug)
2.) Transfer Case fluid change (very easy just need a 30mm socket head, uses ATF +4, use good stuff, you only need 1.25 quarts)
3.) Rear differential fluid change + new gasket ($3.50 at Autozone), uses 1/2 socket head, this one might take a few to change out depending on the last time it was done, you will need to clean the cover real good and casing ring itself before you reseal it, rebolt and refill. Most my time spent here for me was cleaning the cover/casing, mine was in aweful shape.... Uses 80w90 gear oil - use synthetic, fill until it's full - almost dripping out the fill plug
4) Front differential fluid change + new gasket (same as the rear for the most part), although this one is a little easier to do.
All 4 of these are relative easy though. If you can do all the stuff you already listed, these should not be a problem.
Estimating $70-100 (pending on brand/quality) bucks total for the gaskets + fluids for all 4.