Probably easier to undo the two bolts that secure the clamps that hold the glass to the regulator. That will free up the glass and allow you to wedge a stick to keep it up.
I have not taken a door panel off a 98 Lesabre but in general, look for any screws under the armrest, in the bottom of the pull cup, under the inside release handle, etc. Once you have all the screws out it is a matter of prying the door away from the sheet metal to extract the plastic clips. If you have everything released but it seems to hang up in the armrest area, try lifting up on the door panel. Some cars (like my Bonneville) have clips in the armrest area that are released by lifting up on the door panel.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)