i hear Klunking and a feel Bucking ?!
 

i have a 90 mustang w/mods listed below.

When im driving my car slow at low rpms, around 1500 i hear a Klunking and my car starts bucking my head back an fourth ?? Any ideas , U-joints Torque Boxs ? please help thanks

Sounds like:

Your clutch is _gone_ and it's slipping hard. Usually slipping clutches will do their thing at low rpms and will stop at higher rpms. It causes the car to jolt hard back and forth like you described.

The clunk could be a tranny crossmember thats ready to check out and the jarring motions are causing the x-member to slap against the bottom of the car. I've had both of these things happen to me.