So just to be clear, if you are at 60mph and put trans in Neutral, engine at 1,000rpm, then allow vehicle to slow down without touching the brakes you feel a vibration?
If you touch the brakes and it vibrates the rotors are warped, for sure, you need to change the calipers when you change the rotors, one is sticking which caused the warping.
If it vibrates without touching the brakes then it could be ball joints, and if vibration gets less using the brakes then I would say for sure ball joints.
If vibration is only felt when downshifting, so using the engine as a brake then could be motor mount or drive line issue.
If you open the hood and watch the engine thru the gap under the hood you can check motor mounts, put Emergency brake on, put in first gear and slowly let out the clutch while watching the engine, engine should move only slightly up on one side, now do the same test but use Reverse, engines other side should rise up but only slightly.
And in either test you should not hear a clunking sound when engine moves up or down.
No, not serpentine belt related, bad fan clutch can cause a vibration but it is RPM based, usually felt when RPMs are above 2,000 and increasing
Last edited by RonD; 08-14-2014 at 11:37 AM.