RPM dip braking hard AND high speed vib.

Vibration did they index the drive shaft when they had the trans out for a clutch. The idle issue, your throttle plate most likely needs to be adjusted for off throttle position when warm or you have a lazy Iac (idle air control)

majority of times the vibration is the drive shaft so i would check that.. and your idle is probably your iac.. just pick up a new one from your local auto parts store...they are cheap.

20yrs in viberations specialist.dont always believe what u read.your driveshaft was not balanced with pinion flange hench anyway is correct.any change in vib thier would indicate problem from thier back.a 2 piece shaft to doesnt have a oneway fit needs index on shaft itself .also note a true driveshaft vib is 35t65 and arnt normally hi speed vibs.objects that remain a constant speed vib.wheels are mostly 99%.in your case sounds i look at out of balance pressure plate .im not saying your shaft is not at fault .the shop could have damaged your shaft or did they do any work on it?