Had the same basic symptoms.
New track bar, didn't do anything for me. Looks pretty though. And my old one was really old so whatever.
Removed the control arm bolts one at a time, cleaned them up, reinstalled and torqued to spec. Helped a bit with overall jitters, but didn't stop the shimmy at freeway speeds.
Rotated tires front to back, helped some. Took tires to be balanced by someone I trust, he did find that none of them were perfect, with a few being way out. Probably why the rotation had at least some effect. But still, didn't completely solve the issue.
Ditched the original steering. Replaced with the ZJ upgrade, including new drag link and stabilizer. That fixed it. If your steering is original just replace it. It needs to be done even if that doesn't fix all your problems.