65mph Vibration Troubleshooting? Long Post

[dguy210]

I have a 1984 300sd with 325k in which I am trying to troubleshoot a newish 55-65mph vibration. I'm trying to figure out what the next most logical thing to replace/look at is and I have a few curious symptoms.

So far I have replaced:
Left and right tie rods (passenger side boot was trashed)
Both Guide rod mounts (Drivers side was pretty trashed and I had the clunk, surprisingly although the boot on the passengers side was torn the rubber was still in good shape all around, the LCA bushings looked fresh, so I believe the whole arm was replaced by the PO, LCA bushings on drivers side seemed in really good shape also).
Idler control bushings/bolt (used the idler arm repair kit).
Brake pads, front and back

Checked but did not replace:
Upper control arm boots are intact, and the rubber looks decent (no cracks).
Ball joint boots are intact, and the rubber looks good (no cracks).
There doesn't seem to be any real wheel/hub movement with the car jacked up and me prying/yanking on it. I have not repacked the wheel bearings and this is on my soon to do list.

Other Symptoms:
The vibration only seems present when I run 15 inch wheels on the front (tested before I replaced the guide rod mounts), strangely 16 inch wheels (on the front) have no vibration at any speed. I rotated in different combinations, several different 15 inch wheels (different style rims and tire brands) and although they altered the magnitude of the vibration the speed when it started was the same.

I also have a single clunk coming from the rear at low speed with bumps and sometimes stops, but that may be the exhaust or crap in the trunk. I think when I last looked at them the rear sway bar links were ok, but I will recheck them when I swap the tires (see below).

Other factors I will check soon that may be responsible:
I'm running 15 inch rims on the front (michelin tires), and 16 inch rims on the back (normal profile, but nice and wide m/s) the rear 16 inch rims have 20mm billet hub centric wheel spacers made for this application. I really like this look, as you can't easily tell the size difference by eye, and it puts the nose down a little. I'm going to switch these out as my first step, by replacing them with 15 inch all around.

About the vibration:
Comes up at 55mph, is noticeable but not real bad. Before switching out the rubber parts I had a steering wheel shake as part of the vibration, that has disappeared. I just finished the guide rod mounts up a couple of hours ago and test ran it. The steering/handling while not bad before, is really nice now But I still get the highway speed vibration, and this is on a really nice recently paved lane. I'm not sure now though if the vibration is in the front or back, I slowed the car down almost immediately after I felt it vibrating so I can't give a good sound/feel description. I will try to test run it tomorrow with different rear wheels and give a better description if the shakes come back.

Low probability stuff:
I checked the flex discs when I changed the b2-piston last year, I seem to remember the rubber looked pretty new. I don't remember if I checked the support bearing but since I didn't change it, it probably was fine (guessing?). I'll be checking these again just in case, but this seems unlikely.

I have ALOT of tires/rims so I will try switching them around first to see if that cures the problem. I was kinda bummed that after all the work to replace the guide rod mounts and the tie rods, that I still had this shimmy.