If you can't get it loose with some penetrating lube, I think your only option is to start removing the rear axle and inspecting it from the inside. You can give the axle bearings a quick check by lifting the rear end off the ground and giving the axle a little wiggle, you should be able to detect a little movement if the bearings are toast or it could indicate that the axle nut is backing off.
__________________
Yours truly, sincerelly, love always, Josh aka The DirtDemon.