I feel your pain there... I think most can relate to the Jeep thing... as my my XJ is my DD as well.. the thought has crossed my mind just because I don't have the time to be pulling her all apart every time something hits the fan. And with an older Jeep.. I've learned that it's quite often.
If it was a 2nd vehicle for me and not primary... I'd be happy to tinker all day to get it just how I want it. I might feel different tomorrow... I usually do with it...but right now she's really just pissing me off. lol. If I didn't also have a motorcycle and decent weather for it, I'd be screwed right now.

 

You could take off the oil pan and start pulling rod caps off to verify a bearing or piston pin problem or put a new engine in it.

I hear you though, both those options are either time consuming or expensive.

Xj's are not the best dd imo. Let's face it the newest one's are 17 years old.

They are noisy too, even when nothing is wrong, ask anyone that owns one.

I'm not saying there's "nothing wrong" w/your xj.

I wouldn't want to break down on a toll way or interstate in Chicago.Anybody that lives in these parts knows that's really not good.

You'll probably still buy another xj though Lol!

 

The parts they replaced were not guides. They were rocker arms and bridges. Actually, after they replaced them, it sounds a little worse. When I brought it in to them, there was no check engine light on, now it is on and they got oil everywhere inside the engine compartment and on the outside. All over the exterior of the hood and front fender as well. I should have known better. Can't get emotional with these older Jeeps. It is like betting on the Chicago Bears.

 

Actually they should have replaced the rocker with the pivots that they ride on, as well as the bridges? That is good work and worth something, should make slight improvement. But the pivots are critical, is what the rockers ride on. Mismatching with the old ones..well...kind of negates the whole thing. Should have done pushrods as well.
Putting on the bridges requires a skilled hand with a torque wrench, they have to tighten alternately or they will bend as one goes down further that the other (intake exhaust). What this does is misaligns the rockers so they dont line up with the top of the valve quite straight.
On reading this again, oil on the fenders sounds like they were trying to adjust the non adjustable valve train.
If I were you I would ask for the old parts back. If they come up with them, then take it back down and check the bridges for alignment. You can take them back off, one turn at a time alternately, straighten them out, and put them back on, one or 2 turns at a time until you torque them at 23 ft pls, if memory serves but check the spec.