I have a 94 Concours,106,363mi, awesome car, and when I am going about 40-45-50, holding my speed, the car chakes. If I am accellerating or slowing down it doesn't shake. It has done this since I have bought the car. I had my transmission rebuilt and it still does the same thing afterwards. The transmissoin guys told me it is running and anything they do from here would be extra.
I keep premiun gas in the car and it doesn't feel like it is trying to shift gears or anything, it feels like the wheels are out of allignment/balanced. Everything else is ok. Fluid is ok..

What do you all think?

I would start with a wheel balance.

I did a wheel balance and had an allignment done. If I am coasting at 45 it is ok, if I am doing 45 and I push on the gas to maintain that is when it shakes. Kinda feels like it is cutting out but the car doesn't die. I know it is weird.

Plug wires? That's kind of how mine reacted when my plug wires went after the fuel rail recall. Is it only at light throttle before a down shift. OK after a downshift or WOT?