I had this Jeep donated to me (1996 Cherokee Sport Manual 4WD) and the previous owner said no mechanic can figure out what the problem is. Here are the symptoms:
Bucking - Car bucks/jerks. Seems worse at 1st, 2nd and 3rd gear. The car acts as if you took your foot off the gas pedal and then quickly put your foot back on it, you can hear a clunk under the car as if something is getting engaged and disengaged.
Flooring the gas pedal seems to temporarily fix it. If car is bucking and you stop, car will idle erratically, RPMs dropping down to 500 and car revs to 1500 then drops to 500 again. Car idles fine when not in bucking mode. Car seems to buck first thing in the morning. Highway driving seems to aleviate bucking.
If it were mine, I'd spend $100 at a shop that can do a proper complete diagnostic of all the sensors and systems. Then you will know what to fix. I had a similar issue with an engine miss and the computer diagnostic service pinpointed the problem after several mechanics couldn't.
I am assuming that if you depress the clutch, the "bucking" stops??? Sounds like either a TPS mabey or something in the drivetrain is binding up. Need more info?? What all did the mechanic say and or do to your rig before you got it??
Life's too short so always do the right thing even when no one is looking and never look back!
my 94 jeep cherokee 4.o 5speed stick does the same thing sometimes. Mine however does not do the idle problem. I do however notice the bucking especially in 1st gear. I mean Ill let off the clutch nice and easy, and then the u go to give it some gas, and the thing is like riding a bull. Also when going down the road, in almost any gear even on level ground i will let off the gas and the jeep feels like there is so much play in the drivetrain, between leting off and geting back on, there is some kick back. The only time u dont notice it is going down hill. I have 186XXX on my jeep cherokee xj. I have about 3/4 inch or so give or take in the driveshaft when twisting it back and fourth when stopped.
I plan on doing the clutch over sometime soon. I know it has a rough spot on the flywheel it seems when engaging. So new clutch and flywheel are in order. Also plan on going with new HD leafs and skyjacker nitro shocks, and coil spacer up front with new bushings all around.
Might check/change the fluid in the transfer case.. Mine started bucking this weekend, and I was hearing (and feeling) thumps from the undercarriage.. Didn't have any idle problems, though..
Anyway, ended up being that the transfer case was trashed.. I think the bucking and jumping was either A) teeth getting rounded off the gears, or B) broken teeth/various frag passing between the chain and the gears.. Eventually, it just started making a horrific, blood-curdling grinding noise and wouldn't go forward or backward except in 4LO..
The good news is that $356 put it back on the road with a boneyard tcase, and that included parts and labor at a mechanic's shop.
I used to have a manual Jetta before my xj and it had this same problem. im not entirely sure about the manual xj since how mine and all the ones i know personally are auto. but it turns out there is a sensor called a WOT switch (wide open throttle) on my jetta that would change that air-fuel ratio when the throttle is fully depressed to avoid blowing up the engine. fixed the switch and TA DA the erratic idle was fixed and it drove like new again. hope this can help in some way.
p.s. it just seems to me like the description of your problem seems more like a "proper engine running" problem more than a worn out clutch/transfercase etc whereas you havnt described any wierd/loud/scary noises or hard shifting or clutch slipping symptoms.
1) the 96, 5-spd manual 4x4 is probably tops among xj's to own.
2) bucking like you describe could be few things. try in this order:
replace fuel filter. its a $12 part and 10 minutes of work. you need good fuel volume and pressure for good idle and driveability.
there's a fair chance the tps has a worn out spot in it. how to check very easily: get the motor warmed up and disconnect the TPS connector. the check engine light will come on, but should drive fine. if bucking stops, TPS is worn out and is a normal replacement item around 100-140k miles or so. reconnect it when done.
if it's not the tps....the next up is the idle air motor. this is the most likely suspect with the crazy idle where the idle air motor picksup some engine load when you first let out the clutch in 1st gear. use a torx 15 and remove it, clean all the gunk off it, and re-install. then, if it still bucks, get a decent idle rpm and disconnect the IAC (idle air control motor). drive around . if bucking stops now...then you still have bad carbon residue or a worn out IAC .
look, it should be an easy fix, and you do have a rare, great 4.0 xj. with a high flow muffler/cat/ MSD blaster coil/K&N air filter, some overdue fluid changes/plugs/wire cap rotor...you will have an awesome daily ride. it won't cost much, and is incredibly tough.
odds are also very good that the original fan clutch is worn out ($40) normal wear item as well, and is #1 reason for running too hot above 210'F.
here's the biggest tip of all: stickshift 4.0's are known to shear off the motor mount bolts going in to the engine block on the right side, below the ignition coil....if they are loose, replace them/check them!!! these would be the plate that goes against the block, on each side, just above the oil pan. I sheared all 3 a month ago and only wish someone told me the same to check them, there are postings of 95+ year models (only with stick, around 5% of 4.0's have stick!!) that the bolts vibrate loose and shear off due to the high torque output of these motors. if you feel a lot of vibration in the stick (left to right) while going uphill in 1st gear or 5th down the freeway...YOUR BOLTS are loose, trust me... there's a huge difference in smoothness when the bolts are correct, and much smoother idle/freeway feel.