Let's remove the ranchers from the equation for a second, and discuss where the federal government got its claim to the land. I don't think we've adequately explained where their supposed right to this land comes from. We can obviously rule out the idea that the U.S. government homesteaded the land.
Some people have mentioned the Mexican War, which rests on the assumption that the U.S. government owns this land because it violently took this land from the Mexican government and paid them a pittance for it, but this fails any kind of logical test. I'll go back to my computer, if I had simply walked into the store, punched the sales person in the nose, given them a $10 bill and walked out with the computer would I have a right to this computer? Obviously not. So how does force create a legitimate claim in one scenario, but not in another? Furthermore, it's not clear that the Mexican government had any legitimate claim to the land in the first place, because chances are they simply used force to take it as well.
However, if we accept that force is a legitimate means to acquire property, then what exactly is the problem with what's going on with the ranchers now? The Bundys have rounded up a posse and are attempting to take that land by force from the federal government. If we accept that force is legitimate, then the only logical position to take from there is that we have to wait and see which side wins the fight before we can say that they do or do not have a legitimate claim to the land.