Have you tried disabling the rear power lift feature and opening/closing it by hand? If you do, does anything seem stiff or binding? Operating the rear hatch by hand while the power door is disabled will not damage anything.
The rear power door is disabled or enabled through the settings page on the main screen of the van.
FYI if you ever need or want, disabling the power sliding doors is also an option. There's both a dedicated power switch and it can be disabled/enabled through the screen settings. When disabled, they operate purely mechanically.
I don't have a 4th gen (hybrid) like you, but when you disable the power doors, they are extremely easy to move mechanically. Note I used a single finger to open and close the door. I made this video for another thread when someone mentioned they had to force the door really hard, to highlight that the issue was mechanical, as the door should not be hard to open or close by hand when the power doors are disabled. This isn't the rear lift gate, but posting here to show you that unless something is broken mechanically, when you disable the power features, they work manually with no issue, exactly as you'd expect them to work.
I also don't have a power rear lift gate, but I would imagine that it would operate just as smoothly and easily by hand as my non-power lift gate operates, assuming you first disable the rear power door in your vehicle settings. I'd give that a go so that at the very least if you don't get the power issue figured out, you can reliably operate the lift gate manually this winter, and not worry about getting stuck with it open in a blizzard.
