sputter and backfire-4.2L w/ 4.0 head

i just started driving my xj again and now it has some major issues. at about 1600-2000 rpm the engine coughs and sputters with the occasional backfire. it has the 4.2 block and 4.0 head and 24lb/hr. injectors. there is no cat and it has a slight exhaust leak that will not go away! i just put a new gasket on the manifold because i thought the intake was leaking but it still did the same thing. the engine has about 4000 miles on it. i bought it off a friend who rolled his rig so i put an 88 intake and all the sensors onto the engine that was in a 95. i checked the plugs and they were extremly white. so i built an adj. MAP sensor and it is running better buy the problem is still there. the map sensor now has about 7.2 volts running to it which is extremly high, since stock is about 5 volts. i checked the plugs and they are slightly black but when i turn the sensor down it just runs worse. i have been over every sensor, reading every manual and checked for vacuume leaks. i have asked everyone i know but i have came up with no solutions.
PLEASE HELP ME SAVE MY JEEP!

O2 sensor? Has the distributor been indexed?

02 sensor is fine, and as far as indexing goes, WTF? i thought the distributer could only go in two ways, 180 degrees different? it seems like its fireing right and if it was missing wouldnt it be threw out the rpm range?

It's in the check list for bucking, surging and popping out of the intake. Book says many Renix motors, should re index the distributor if they display the symptoms you described.
It's also recommended to re index the distributor after a cam change. Having a 4.2 bottom end, I think, qualifies as a cam change.
Not to hard to do and something to eliminate before you move on to other stuff.
Typically bucking at mid range or off idle, is a lean running motor. O2 sensor and/or connections. Timing, distributor index, TPS, low fuel pressure (or volume). I'm guessing, but a faulty cam position sensor, might also cause lean at speed, because the fuel puddles in the intake.
Mine was doing what you described right around 2000 RPM, white plugs and bucking. I re indexed the distributor, drove around with a fuel pressure gauge tapped to my windshield wiper and did the same with a vacuum gauge. Turned out to be a combination of O2 sensor and resistance in the connectors between the O2 sensor and the ECU (and the grounds). My O2 sensor was weak and combined with the resistance in the harness, caused it to barley work. I also found some resistance in the MAP circuit harness, which probably wasn't helping much.
Good grounds are important.
Did you ever read through the 4.0 head onto a 4.2 at http://www.off-road.com/jeep/new/40conv.html They might have some usefull info.

yeah but im still not getting the indexing thing. the distributer can only go in two ways, and one of the ways is 180* backwards, so it wouldnt start, right? and i have changed the O2 sensor and it didnt help any so i returned it and got my 60 dollars back. i just put a new pump and filter in it and im getting the proper fuel and volume (24lb/hr injectors). there has got to be some thing else that would cause it to run lean like this. would the CPS cause this?

7.2v is a major problem. Verify you have the ground from the head over to the firewall. If you still have 7.2v, the ECU is hosed. It has a 5.0v regulator that is used for the sensors.

Re indexing: you have to remove your distributor and turn your oil pump to the designated direction (commonly found in your service manual) to adjust your timming.