my 90 gtu was jerkin really bad, then i ordered and installed some mazdatrix performance engine mounts bout a week ago and it helped alot but will still jerk when i let off the clutch fast and when i press the gas and let go do you guys think its the differential what other possibilitys could it be and should i get the rb torque brace?

 

If you did a torque brace you're really just putting a bandaid on that is not fixing a bigger problem if it would even fix it at all. If it feels like it's in the driveline then I'd look at the transimission mount first (it's a pretty easy one to check) and then worry about the differential. I knew the top differential mount on my turbo 2 went out but I couldn't really tell without removing it. With that differential mount out, the car would make a "thump" sound when I got on the throttle.

 

Could it be the dashpot?