This also happened to me with '73 280 (114)

Pulled the door panel and found that channel controlling window up-down had merely dropped one of its supportive bolts..... making it wider at bottom than top, causing the window to sag below normal down position, off ballance. Recovered the bolt from bottom of door and secured it with locktite.... an easy fix that requires no special tools.

Once you have figured out how easily the door panel is removed, the rest is simple too.

You should probably tighten all other bolts and screws while you are in there - also grease all moving parts, except the mechanism that holds the door open.... you want this to grip and not let the door slip shut.