I have just bought a 1993 mustang gt, it only has 20k miles on it, and i put a new clutch in it....but around 3 to 4 rpms the car vibrates like crazy.. I have a strong feeling it could be my armonic balancer,but you can hold the engine and it revs nice and smooth so it has to be in my tranny?

 

Does it vibrate like that while you are in neutral and rev it? Did you get a new flywheel or have your old one resurfaced? If it does it while in neutral also, try revving it from under the hood and look at the harmonic balancer.

 

i'm guessing flywheel...but you never know