99 windstar 104k

Sometimes when I let off the gas the van does this vibration/shuttering
thing. Not too violent or anything, I think only the driver notices it.
Seems to do it at certain speeds, around 40 seems to be pretty common, I
cant get it to do it all the time. Recently I've been trying different
things to get it to do it to get a better idea of what it might be.
Yesterday I felt it shift into the next gear at about 38, let off at about
43 and it did it. The first time I felt it was on the highway this summer,
I'd normally cruise at 75 or so but sometimes I'd hit traffic and be in the
60-65mph range and I'd let off the gas to slow down and I'd feel it a bit.
My wife claims it did it to her while accelerating through 40 the other day
but I've never had that happen, only when letting off the gas and it seems
to be right before I reach the point of having no pressure on the pedal.

trannie fluid is at a good level, no foam, even smelled a little tasty.

possible options-

something jacked in the trannie? maybe a downshifting issue?
motor mount? maybe i feel the engine moving because of the drop in torque
but I'd think I'd feel a broken mount in other stages too.
maybe the engine itself is running bad at that point and vibrating?