Remove the door panel and inspect the tracks, and the gear. If the glass came off of track the gear might have stripped.
I just fixed my 98 grand Cherokee with drivers side window movement problems. there was dirt in the window actuator track and pivots. the grease was all dry. i lubed it up with LPS (a kind of spray lube. white lithium should be good for this too). with the vehicle running ( so i got good power, & didnt drain battery,) i ran the window up and down ( a lot) adding more lube every now and then. it is good now. hope this helps.
I have the same problem with my 1999 Cherokee and after removing the door panel found that the plastic bracket on the window regulator is broken off. I purchased a repair kit from Steiger performance. The kit is for Cherokee not grand Cherokee. The link is http://www.steigerperformance.com/