This is common to aerostars- the run channels wear out and need to be
replaced- last time it cost about $60.00 per side.
There isn't any adjustment to the glass mechanism, unfortunately.
You can replace the run channel without pulling the glass or trim panel by
loosening up the mirror and lubing the new channel with light oil or
silicone and working it into place.
Or, just give up like I did and carry a can of WD40 in the van!