Using wheel chicks or If handbrake works well enough to hold car well, engage it tightly and with transmission in low gear, give it some gas and let out the clutch. Have an assistant view rear springs while doing so. Any shudder or other observations of note in behavior of springs or driveline?
Would certainly be better to be on a drive on lift,,,,,looking for unusual engine and transmission lift or rock.
I need to do this test again. I've done it once and I'm pretty sure I saw no shake with that. That's what pointed me to the back end and changing the springs.
