the middle mount is one of the more important mounts, a little bit of a pian to replace but not impossible.
if the vibe was from the convertor(lets say it threw a balance weight off) you would feel the vibe all the time (PRNDL)
you say u only feel it in park and drive. so my guess is the convertor is fine.
I'm in Oshawa so hit me up if you need any help.