Is it impossible for prior service to join a Ranger Regiment?

No one will care or even know from where you came. The only issue I have is where you stated “not wanting to be a grunt my whole career”. Two things, these men are professionals and live this their entire lives. It’s not a job and it sure as hell isnt temporary, it is a lifestyle and mindset that we carry to the grave. I don’t think you understand and you should definitely devote serious time to determine if this is what you want. If you’re not committed to a lifelong change then don’t do it. They will weed those guys out and you’ll find yourself with a failure under your belt on a worldwide assignment. But the other stuff; nobody gives a damn who you “were or where you came from”, it’s all irrelevant. No... thinking about it, you shouldn’t go. Stay in the guard.