Went and visited my sick Jeep today and tried to figure out where the noise was coming from. The stethoscope trick was a great idea, but didn't really tell me much... the noise seemed to be a little louder at the top of the engine but not by much, and where it's really noticeable is underneath... as when I stick my head under the Jeep I can really hear it.
I took off the valve cover (that was fun with a now warmed up engine!), and started it back up again, hoping I could see a difference in how one of the rockers was behaving, but they all seemed to be working the same, and pressing down on them one by one (both on the valve end and the push rod end) made no difference in sound at all.
If the problem was a collapsed/stuck lifter wouldn't it be apparent in the way the rockers move? Just how DO you figure out what lifter is bad when you have a bad one?
The noise seems to... not exactly quiet down but maybe smooth out a bit at higher rpm's, if that means anything.
I love this old Jeep, but it's got 300,000 miles on it, and if it really needs a new engine as much as I hate to say it I'm not sure it's worth spending $2,000+ on a Jeep that won't be worth that when it's fixed.
Thanks,
Jeff DeWitt