Could be the throwout bearing, mine vibrated when it went out. Could pe a pilot bearing too, or you didn't center out the clutch right with alignment tool. Best thing to do is to go under there and check it out. Replace your bearings this time, I know Luk throwout bearings suck, so Id go with some Ford Racing ones. Theyre real good.
1998 Ford Mustang GT 4.6L
40's Flowmasters K n N Intake and BBK Long Tubes BBK X-Pipe
Ceramic Brake Pads 88 Rotors Cross Drilled
UPR Control arms UPR Tubular K-Member
Shark CC Plates Coil Over Monroe Struts
Rear Monroe Shocks Monroe Motor Mounts
Steeda Short Shifter and Steeda Clutch Cable