Theory: I recall that the first time I rotated the Jeep's tires--and this is borne out by what it says in the maintenance log--I got a horrible shimmy at speed. I de-rotated them and it went away, but I did rotate the tires a couple thousand miles before I got the new ones on--same theory, now that I think of it. But WTF, it's been a year, so they're due for rotation anyway, right?
So I take a test drive, and the first couple of times I hit 55 MPH there's absolutely no shimmy whatsoever. "It's the wheels, and it's fixed!" I thought hopefully.
This was on back roads between farms that are not the smoothest; once I get on the highway and haul 'er up to 55, though, JIG JIG JIG, right at 55 MPH indicated. If you did a plot of the intensity of the vibration, I'd bet it would make a very nice bell curve with peak amplitude at 55, and rapidly falling off to zero after 5 MPH in either direction: 50 MPH, none, 60 MPH, none. Yeah, I guess that was too much to hope for. *sigh*
Either A) the former rear pair of tires has exactly the same resonant vibration as the former fronts do, or B) it's not the wheels or tires. Guess which I suspect it is.
Having checked the ball joints, the tie rods, and the track bar, I'm now down to two suspects: the control arm bushings and the wheel bearings.
The control arm bushings themselves are not very expensive--$50 a side--but they are a royal bitch and a half to replace, and once done the front end must be aligned. Entire control arm assemblies can be had for about $100 per side, but again once they're replaced it means an alignment, which is about $80.
This model year Cherokee uses cartridge bearings, which are about $90 a side. No alignment or special tools needed (except maybe for removing the axle nut) but it's two hours for me to get one side apart to check the damned things. On the plus side once the cartridge is out I can also check the axle bearing and the u-joint to see if that's the problem. (If they were open bearings that would be an easy-peasy and cheap fix and I would have repacked the bearings by now and replaced any that were gronchy.)
Checking the bearings is a project for next week, though.