Could also have lost (rusted away, snapped, disintegrated, exploded, imploded, etc.) the brake retainer spring, which would let the brake pads rest against the drum, and dig in when you try to move it.
Another, prob. less likely possibility, is maybe the emergency brake mechanism is failing to release? Depending on your truck model, you may be able to tell if the e-brake releases when the lever is released (nice to have another body to help with this!). If this is a '48-55 6400 series, I can tell you from VERY recent experience that you can hammer forward and backward on the parking brake lever on the back side of the backing plate, where the cable clevis attaches, to help loosen it up.
I don't know that you've got much to lose by being agressive with this, since you'll almost definately have to do a brake job anyway, to get the truck roadworthy, IMHO. I'd say work the truck back and forth with the tractor, nudges at first, then more aggresive as need be. I don't think dragging it will hurt much, other than wearing the tire some, if it's on hard surface.
The RIGHT thing to do would be to take the wheel(s) off and see if you can get the hub off, but that gets rather involved rather fast, especially if you're "out-standing in your field". And BE SAFE! Getting rolled over by a big truck that doesn't even run is a silly way to end up handicapped, or worse.
Let us know what happens. I LOVE hearing about old, ignored and forgotten stovebolts coming back to life!
-Michael