One problem related to transmission gear slipping has been reported for the 1996 GMC Sierra. The most recently reported issues are listed below.
When returning from a trip the transmission started to slip when engaging overdirve. . Over the next 150 to 200 miles it progressively got worse and now overdirve and 3rd gear are entirely gone and the engine just freely revs. . Only 1st and 2nd gear are usable.