First, check that the parking brakes are fully disengaged. Loosen as necessary. Also be sure you have a copy of the TM handy.
I had a similar situation happen a couple months ago and it was caused by the actuator rod bolt being broken and permitting the master to be engaged at all times. I ended up rebuilding the surge actuator and the brakes on both sides. You will want to get that drum off and make sure that you haven't damaged anything internally. Repack the bearings while you're in there (or replace with new) along with new races / seals as required. I wouldn't suggest locking out the brakes in the mean time unless you can verify that the master cylinder actuating rod is sliding freely in the lunette bracketry and not binding in any fashion. Also, back off the brake pads slightly (you may need to loosen the parking brake cables at the handles to get more travel).
http://www.steelsoldiers.com/trailers/44524-m101a2-a3-brake-shoes.html is the thread with the part numbers on the bearings, shoes and seals. Be sure to read all posts in there as there are a couple hints.
While you are taking it apart, check the master cylinder actuator rod, I found mine to have a broken bolt and therefore locking the brakes at all times (it is above the lunette, you'll need to remove 8 nuts to get the lunette off). The master can be replaced at NAPA if you need to (
http://www.steelsoldiers.com/trailers/77229-m101a2-master-cylinder-question.html) I know I posted a few tips on other threads on how to get a new crush washer to fit.
The drums themselves seem to be unobtanium, so be careful while pulling things apart.
I know I haven't covered everything. If you can change regular drum brakes on a vehicle, you can easily get the trailer done. I think it took me about 6 hours from start to finish once I had all the parts in front of me. This also included building a pressurized brake bleeder system.
Good luck on the repair.