Did the shop replace or inspect the clutch when they rebuilt the tranny?
Did the problem start immediately after they worked on it? Or before
they worked on it?

It sounds like the clutch may be wearing out, or they managed to soak
the clutch plate with tranny lube or used too much lube on the pilot
bearing when they were removing/replacing the tranny (which got forced
out onto the pressure plate), or the pilot bearing was damaged in the
R&R process.

Had a 67 GTO one time with a similar problem and it was because I used a
"rebuilt" cluth/pressure plate.  Once I installed a "NEW" one, the problem
went away.  Good Luck

If it is a new or rebuilt clutch, the odds of it being out of balance are
very good.  Contrary to popular believe, they don't balance these assemblies
worth dog doo.  If it vibrates at certain speeds worse than others and it
increases with speed, then take the flywheel, pressure plate, and clutch
disk out and have a good machine shop balance the assembly.

Grease will only make them chatter when engaging unless there is so much, it
won't engage and is slipping.  One way to check slipping is to get it in as
high of gear as possible, drive about 2000 RPM or less, and then floor it.
If the tach soars up and the speedo is not anywhere close to matching, your
clutch is slipping.