i have a 1993 300D...I recently spent a lot of money repairing my transmission...after having to bring it back a few times for adjustments to the shift module, it shifts almost perfectly now..however, at times it still shifts hard, or when it is at high RPM's and i let go of gas, i hear and feel a thump...the trans people say its a vacuum that controls the shifting on this car...is this a legit response from them??? how do i replace this vacuum and how costly is it?? also, where is this vacuum located??

thanks in advance...
Mario