If the HST is "in gear", then the tractor won't role due to the hydro fluid, t'ain't like an automobile tranny with a torque converter.....
Most likely it is the brake cams being stuck in the on position if the gearbox (or hydro) is in neutral. The springs that are supposed to pull them off just don't seem strong enough once corrosion sets into the aluminium housing with the steel brake cam shafts.
Crawl under and give them both a tap with a hammer to release....