I had a shutter in my 99 Expy, shifting from 3rd to overdrive. It sounded like runnning over BOTS dots when placed on the road coming to a stop sign. Drove me crazy so I changed the tranny fluid, including converter and changed the filter.
It wouldn't go away entirely so I emptied out a qt and added a qt of Slick 50 tranny additive. It probably took 25 miles to work it's way in but the problem then went away.
I know many don't believe in additives but this worked great.
Now 285k on the tranny and I change the tranny fluid every 50k with a qt of Slick 50.