MileHigh ... are you in Colorado? I'm in Montana. I also have a '91 XJ with the 4.0L engine and the AX15 manual 5-speed transmission. The gearbox on the AX15 is very good ... strong and smooth shifting.
But my jeep is showing the same symptoms as yours ... difficult to shift into first gear when the jeep is stopped. Mine is doing it because I had the flywheel resurfaced when I put in the new motor. My old flywheel was scored up pretty badly, so the machinist had to take off about 0.080" to get it smooth.
Making the flywheel .080" thinner moves the clutch disk and pressure plate farther from the throwout bearing, so the throwout has to move farther to disengage the clutch. The problem with that, is that our slave cylinder/throwout bearing can't be adjusted. It's designed for a specific travel distance ... no more, no less.
So ... my clutch isn't fully dis-engaging when I push in the clutch pedal -- even all the way to the floor. The amount of engagement is really small ... it took me forever to discover it, but it's there.
Let me ask you something: When the jeep is cold ... especially in the winter, do you find that it's easier to get it into first when stopped at a red light? And then when the transmission warms up, it's more difficult? That's what mine does. The reason is that cold gear oil is much thicker, and the transmission gear shafts are more resistant to spinning in thicker oil. When the oil warms up and flows better, the tiny amount of clutch friction is enough to spin the shafts and make it harder to get into first.
So ... it's possible that some previous owner in your jeep's 20 year history has also had the flywheel resurfaced. If so, that is your problem. You are definitely not supposed to do that with our XJs. Wish I had known that.
I would recommend you bleed the hell out of your slave cylinder first. If your fluid is dirty, it should be flushed anyway. You'll see the bleed valve on the driver's side of the tranny bellhousing.
I bled the hell out of my system and put in a new clutch MC. The slave was already new with the new motor. None of that fixed the problem.
The real solution is to put shims behind the old flywheel to move it closer to the slave/throwout bearing. Or fabricate a shim to put under the slave cylinder/throwout to move it closer to the flywheel. Or just replace the flywheel with a new one that is the proper thickness.
It's a big job, and I'm going to do mine soon. It involves dropping the transmission, or pulling the engine. I'm going to drop the tranny, put in a new clutch kit and new flywheel or shims, rebuild the tranny and T-Case while they are out, replace all 5 u-joints on the driveshafts, and do a new exhaust because mine is rusted. Big project.
