Regulator or Weatherstripping?

[RocketTech]

I have a 98 Expedition 4x4 EB- the Driver's side window will pop-out of the channel when closing the window while moving, sometimes with just the blower on high. Seems to pop-out when almost closed, as it is a tight race to avoid having my fingers crushed while holding the glass in and closing.
Do I fix this by replacing the window regulator or the weatherstripping? The window seems to 'lean' forward, so the rear window channel is not guiding it. Any help is appreciated.