What rank would I be at if I rejoined?

If another branch would accept you and you were simply trying to enlist, you would enter as an E-2- E-4 depending on the branch. But you might be able to find some way to transfer your commission to another branch though this is unlikely especially if you were out for a significant period of time. And any time you "owe" the Marine Corps in the Reserves would be eliminated by service in the other branch on active duty.