Due to character limitations am adding.....There are two shifting solenoids, one of which, controls the 3-4 shift. I don't recall, off hand, which it is, but they are noted as "A" and "B". There are lots of YouTube Vids on this very popular tranny. Good Luck
Ok, I'm a new guy here, so be careful what I offer...LOL! I have a '99 "Burb" w/ a 6.5 TD and 4L80e tranny. The first, easiest and cheapest attempt, add a can of Seafoam TransTune, available from most all auto supply. Follow the directions. Does the seller offer any services, that is, fluid...