The window on my '95 Jimmy would stick to the rubber gasket and make it act like it was the mechanism wasn't working. Take a metal paint scraper (suitably lubricated with WD-40) and slip between the bottom of the window and the tailgate. While pulling a little on the scraper handle (don't go MONGO on it.....(Watch Blazing Saddles if you're NOT familiar with the term MONGO)) repeatedly press the button on the tailgate. It should release.
If it doesn't, you'll need to get inside and remove the interior panel for the tailgate. (I just remembered 98 and up had a 1 piece flip up tailgate rather than separate tailgate and Window.) Remove the Interior panel. The Latching assy is right where the Window loop inserts to the tailgate.
Remove the mechanism and spray PB BLASTER (similar to Liquid Wrench) on the latching mechanism, to soften old congealed grease and caked on dirt and debris. Clean mechanism as good as you can get it (Brake cleaner has enough force to blast the crud out. Use Non-Chlorinated version.) then coat it with White Lithium grease and exercise it manually 10-15 times. Install and verify operation manually tripping closure. Connect electrical and test again.
A tube of Di-Electric Silicone grease skimmed very lightly on the rubber gasket for the rear window will keep it from sticking further.