Two things I would suggest.. I've had similar problems with mine over the past.
(1 easiest) get some thick tire silicone (black magic or some ture shine with heavy silicone) at auto zone or a similar store and coat the seal on the top where the window goes in.. alot of times that rubber seal becomes really dry, or gunky and has a tendance to snag or slow a window.
(2nd) try removing the panel on the inside of the tailgate you should have a clear view of the track... if its carpeted, pull the carpet the panel will be behind it.. Get some good grease and **** that bad boy up.
Those two together should set that window free, if not you could be looking at a motor replacement.. but that doesn't sound like the case...also, it doesn't sound like the track is messed up.. usually when that happens I've noticed that the glass will tilt or not go up at all..etc.. try the two I suggested.. they should work fine.

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