Re: Stupid transmission question
Sounds like the throw out bearing to me. The throw out bearing is the bearing which the clutch fork rides on. I could be getting this slightly wrong due to I don't know the exact terminology, but basically when you press the clutch pedal it causes a fork to move forward and depress the clutch. This fork rides on a bearing, called the throw out bearing. These sometimes go, and when they do they cause noise and vibration when the pedal is pushed in.
I had to change mine on my 95 YJ. If your YJ is a 95, then you should be in luck (somewhat) because at least you have the external slave cylinder kind rather than the internal slave cylinder kind, which would make things more complicated. Basically you unbolt the bell housing from the engine, push the tranny/xcase back (after removing driveshafts), pull off the clutch linkage, remove the old bearing then replace it with a new one and put everything back. It sounds simple, but it actually is a real PITA if you don't have a transmission jack. I used a come along tied to a chain run across my roll bar, but suffice it to say that supporting the tranny/xcase (which is REALLY heavy) while you are moving it back and forth is difficult without the right equipment. Also, it's a pain to line it up (without a tranny jack) to get it back in because you have to get the input shaft lined up just right to get it back in.
I did it myself with the help of friends who've done it before. If you don't have similar friends, you might want to pay a mechanic to do it.
I'm not sure what the plastic sleeve is that the mechanic is referring to, but I did hear of a similar thing happening to my friend's 95 yj (4 cyl), so I don't think that it is necessarily off base, just I don't have direct experience with it. But it is most likely that in order to get to this sleeve the mechanic would have to go through the same procedure to seperate the bellhousing from the engine, which is a PITA and/or requires a tranny jack.
Find someone who has done it before and get them to give you a hand.