And I Def don't think you are a whack job or have shifty opinions. People on here know these cars.
You're nice. I'm wrong more than I'm right. It could be a lot of things. I just like to start simple.
Used manual trans high torque vehicle having non performance issues? Driveline is my first suspect. Wheel bearing, tie rods, ball joints, axles, clutch. Motor mounts sound needed, but in my experience a revved up motor leans instead of shaking with bad mounts and only really imparts vibration (not an absolute by any means).
Front end is easy to diagnose usually.
- Jack up front of vehicle
- Shake tire left and right to test for tie rod play
- Shake tire up and down to test for ball joint play
- Listen for obvious humming from tires while driving for wheel bearing
- If the boots are good then check for any play. If none and it doesn't click while turning, you're fine.
If you made it this far, you're looking at clutch/trans. Shaking / shuddering initially implies warpage (usually from heat) or rounding of a bearing/guide. Obviously the input shaft and gears are ok, because you can drive and shift. I'd guess pressure plate warpage because it's the most simple answer.