Originally Posted by big john

I would start by trying to find out why it's idling so fast. Might have a vacuum leak. Normally problems with the way the things run isn't going to affect deceleration but if the idle is surging you might feel that.

If by shuddering you mean like a mechanical vibration that sounds like a transmission or final drive issue. Maybe even a motor mount. Even a timing chain. Check for a vacuum leak first to try and get the idle squared away.