2001 Chevy Prizm Grinding, shivering car

I just had the exhaust fixed on my '01 Chevy Prizm (literally, just the dents beat out to quiet it down), and now I'm hearing a bunch of stuff - I'm not sure if the problems are new or I just couldn't hear them before. Once she's warm, when I'm idling, there's a LOT of vibration - more of a "shiver" than a "shake", and a kind of grinding noise. I'm also dealing with sluggish acceleration and, when accelerating at low speeds (like rush hour morning traffic), she sometimes lurches a little, and I sometimes get a lot of grinding before she gets in gear. No check engine light since I got the exhaust fixed.

Someone told me it might be fuel related, so I got the fuel system flushed, but it didn't help. I was also told the serpentine belt is going bad, but they had to order a new one and I won't have that put on until tomorrow - any chance that'll fix what's going on? If not, any idea what might be causing this? Thanks for your time!

Tina