could be a frozen park brake cable
also could be a misadjusted wheel, self adjusters are not always 'self adjusting' or they tend to over adjust and tighten up if hardware is not right inside.
then again they might just be worn out so badly and hub is grooved out and the shoes are just stuck inside the groove while they are adjusted up.

if you can get the wheel to turn as you say, then try this
these trucks do not adjust from the backing plate side... you have to pull the wheel and get access to the adjuster from the hub side through a hole in the hub so.....
pull the wheel, get a prybar or something to put between the studs and force the hub around till the access hole is lined up with the adjuster at the bottom.
Then deadjust the wheel as far as you can get it
the wheel should come off then.

A note: about the park brake cables...
they connect on the back side of the backing plate to a cantilever/essentric style splined stud, you can just unhook the cable by loosening the bolt on the yoke assy and slide it off the stud at that point and relieve the pressure from the stuck cable (if this is the case)