jeep running problems

my jeep has issues.... my xj starts just fine and has a nice idle. It will even rev in park and neutral. If you wish you can go in reverse. But... if you want to go forward ... spfff... it dies. I'm lost. We are looking at vacuums lines, and electrical but i really dont know what im looking for... any ideas?? The Jeep is an 1990 xj with the 4.0.

the thing that started all this was the jeep would hesistate and almost drive in park then we gave the engine a tune up and it ran better but then died two minutes later after my buddy drove down the road and turned around. Since then we replaced the cps sensor, the ingnition coil, bastil resistor(or however you say it) , and we are in the process of checking grounds and wires that shouldnt be grounded that are grounded and/or some sort of vacuum leak. Any ideas or help would be greatly appreciated.

correction

correction from previous post: it would hesistate and almost die. The jeep is running right now but will die in drive

We just had that problem two months ago with our 89YJ, drove us nuts as it was intermittant at first then did it all the time. It turned out to be the neutral safety switch in the tranny. We jumped it at the connector with a ground near the firewall so now it starts and runs in any gear. 89YJ, 4.2L, auto tranny. What a PIA that one was to find.... Ran every single snap-on diagonstic system he had on it, looked like the jeep was on terminal life support with all the wires coming out of it to the machines.

is there any other options?

we have consider the neutral safety switch but we havent messed with it yet but just to keep my options open is there any other thing that might be affecting the truck while in the drive position

Not that we found, I had just done a major conversion that removed the carter BBD and replaced it with a Weber, did a nutter bypass on the computer that took it out of the ckt and left only the ECU controlling the engine, put a mechanical fuel pressure regulator in to set the fuel pressure at 4 lbs. Removed and cleaned EVERY ground in the vehicle. Removed and cleaned EVERY connector, replaced the hall effect pickup in the distributor which was borderline. New cap, rotor, wires and plugs. Replaced EVERY vac line and hose from bumper to bumper. It STILL died. Finally strapped it up to the shop 5 miles away and used the diagonstics tools, we got lucky.

I hope I get lucky tommorow