The issue at hand seems to be a stuck roller that came partly out of it’s track, jamming it in place.
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https://www.uhaul.com/Tips/Moving/About-U-Haul-Roadside-Assistance-20869/
If you can’t get it back on track yourself, or you aren’t confident you can do it without damaging your truck, check that out
Assuming that pushing it up and pulling it back down doesn’t work, and the track is clear of obstructions…
There’s a help (roadside assistance?) phone number in the little receipt packet they gave you.