I own a '94 Z28 400 HP L1 with stock clutch. Hammer down, I jammed 3rd
gear. Clutch won't disengage now & I can feel vibration in the clutch pedal
with my foot resting on it. Tranny still shifts. Any ideas? throwout bearing? Pressure plate? Slave cylinder? Clutch has about 4k miles since complete rebuild.

 

That will teach you. Check for leaking fluid, but chances are that whatever you did will require removal of the transmission in order to get a good look and see what is causing the problem and fix it.