Edit: Before you try the instructions below, get a bolt or punch that will go down inside the clevis and use a hammer to try forcing the rod backwards toward the housing. Be careful you do not damage any threads, if rust is the problem that may be enough to free it up. A couple squirts of penetrating oil in the opening on the clevis may help, but if you get it on the brake band you may have to remove it and clean off to have any brakes on that side once you get home.
Unfortunately you will need to remove the final drive to do anything with it. Once it comes out about half an inch and clears the dowels nothing holds up the end. Jack up the left side so the wheel barely clears the ground, then WEDGE THE FRONT AXLE
. Unbolt the disk form the hub and roll it away, caution, if it has a weight or fluid it will be very heavy (125 to 400 pounds depending on how set up). Then remove the pin in the brake rod. Remove the bolts that go through the fender along with the ones that only go through the final, bolts are on top and bottom. Once that is done, it will simply slide out though may require a little persuasion if it has not been off in a long time. It has a shaft approximately 12 inches long that extends through the oil seal into the differential gear, so you need to come straight out. It weighs about 75 to 80 pounds, and can be done by hand, but a second person or a rope block tied over head helps. I have done it by myself a few times, but now have a bad back as a testament to doing that kind of thing to much. I assume it is the pivot type brake, so once it is off a little penetrating oil and gentle persuasion should free it up. Putting it back together be careful to go in straight so you do not damage the oil seal. Here is a picture of the parts inside the housing. One note, the brake drums are not shown, but while it is rare, there is the possibility it may be broken and the parts wedging it. If so, just cleaning out the loose parts should free it.
Once you get the problem solved and back together, remove the front axle wedges to prevent extra strain on axle and bolster before lowering the wheel.