I finally fixed it!!! There was a short thread I came across here about a couple of weeks ago about someone's father's car that was experiencing the same issues mine was, a vibration in the 2500 rpm range regardless of speed. They discovered that some of the bolts that connected the bell housing to the engine were missing! That gave me new hope so I looked all around and found that when the transmission had been replaced about eight years ago, they had gotten the wrong lenght bolts in the wrong holes! The top two require shorter bolts, they used the long ones and consequently the bolts didn't seat-they still had about a half inch to go. I'd been experiencing some odd electrical issues that were solved by adding an extra ground strap on the engine. That fixed it so I didn't give it any more thought. Turns out the bell housing bolt that is also used for the main engine ground requires on of the longer bolts, they'd used the shorter on so it only had about one turn's worth of grip and had backed the rest of the way out! Got all the bolts swapped out and now its as smooth as glass, partly thanks to all the stuff I replaced but mostly because now my tranny and engine are properly connected!
