Starter that won't disengage

Long story short, my uncle has a 1979 Power Wagon Palomino Edition, 360, 4 Speed. He says that when he turns the key to start it that the starter will not disengage and continues to run. Said he had to disconnect the battery to stop it. He says it does the same thing if you try to start it by jumping the starter relay.

We are 6 hours apart, so I can't just check it out myself.

I say it's a starter problem.

Any ideas / advice that I can share with him?

I agree

rusty shaft, or broken spring, arm, or some such.

Bad solenoid.

might also be loose and binding up