I'm currently battling the same issue on my 02 GTP, at 30mph under light throttle when the TC engages I sometimes get a shake in the front end. It does not feel like torque converter shudder, feels more like a misfire. Have you checked for vacuum leaks?
Check the two plastic T's, one under the snout of the S/C and one on the fuel rail. They tend to crack or break and can cause the engine to run poorly and shake under load. I checked these on mine and also did a full tune up, however I think my motor mounts are the contributing factor.
To summarize, start by checking the plugs/wires/vacuum lines/etc
also check the mounts
have you changed the trans fluid + filter?
