Welcome to the Forums! Probably the best place to start is having the van scanned for codes incl transmission codes. The P1811 long adapt shift may very well show up.
Have you serviced the transmission lately with a new filter & Dexron VI (6) fluid change?
This also allows inspection for any particles inside the pan.
When my Buick was acting up and had hard shifts, I added a can of Sea Foam Trans Tune. Trans Tune - Sea Foam Sales Company
which really helps to remove varnish, drove around for 750 miles or so (which did make shifts smother) and then did a shift kit and new fluid & filter in the 4T65e. All is well now.