drivetrain vibration mystery

I have a 97 3.5. A few years ago something in the drive train started vibrating/resonating strongly when the engine RPM's reach about 1800-2000. I can feel it most in the gas pedal but it also resonates through the dashboard and sometimes makes the dash cover buzz. 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. Anyone ever had this problem?

Thanks,
Mike

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

Anybody come around here anymore? I guess these cars are pretty old, few and far between. 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 replaced it and now there is no more vibration or resonance, smooth and quiet as can be. Makes sense why it was worse with a/c on.