Go get a small garden hose, from inside the cab, pull the rubber flange down (on the gasket), and insert the hose there, and then continue to move around the window pulling down the flange and using the hose to hold the flange out. Then once you have 1/2 - 3/4 of the gasket pulled back with the hose, you will be able to gently push out the window and gasket together.
(edited, I meant 1/2 to 3/4, not 1/4.
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