drivetrain vibration/resonance

I have a 97 Concorde 3.5. A few years ago something in the drive train started vibrating/resonating strongly when the engine RPM's are about 1800-2000. More intense with a/c on. It didn't start out faint and get progressively worse, just started one day and has been there ever since and hasn't really gotten worse. Actually started after replacing my plugs, wires, intake gaskets, and valve cover gaskets. I originally thought engine, then I thought maybe it's in tranny, but it's worse when a/c is on although compressor is quiet and works great. Any ideas what it could be?

Thanks,
Mike

More info: has 198k, runs like a champ, no hesitation; tranny replaced @ 104K

So it looks like nobody gets on the Condorde forum anymore...anyhow, on the slight chance that someone else has this problem and could use the info, I wanted to post that I've figured it out. I discovered that one of the a/c bracket bolts was missing, the one in front next to the timing cover. I got one and put it in, and BAM - no more vibration or resonance, smooth and quiet as can be. Makes sense why it was worse with a/c on.