Oh yes done this when the window started flopping around.
Turns out the guides that run along the rail chanel had crumbed way.
Took off the door card and pulled out the window mech, then the window, which you have to pull out through the top.
As said unbolt the guide rail and don't spend 30 minutes getting frustrated trying to find a way to slide it out upwards cause that don't frigging work. Drop it down and out through the door hole.
All I did to repair it was slice some 10mm clear plastic tube, vertically, then screw it to the guide rail both sides. This tubing rolls inward on both sides and leaves a gap wide enough to hold the window.
To finish all I did was lube the plastic tube with olive oil and Bob's your uncle . . . well if he has the operation he can be your Aunt ! !
10 bucks in plastic tube and another 5 minute job done again . . . . YEH BULLSH#T !
My love for Skylines knows no bounds !