I saw lots of threads on Detroits with a mar-tac, but couldn't see anything about a lock-right needing or not needing a mar-tac.

Since I have the front apart, now would be a good time if I need it...

I didn't think I needed one, but as I was staring down the axle tube, I didn't see anything blocking the view to the end of the long side axle shaft... kinda surprised me as I thought there would be a pin in there or something...

Yes, you need either a Marlin tack or the outer C clip on the iner axle. The reason you don't see the pin is that you probably installed the caps that go over the ends of the C-clip rear axles. You don't need these for FF axles, but it won't hurt either.

If you want to remove the inner circlip from the Birfield then you do have to run a martack. That is what I have. Longfields and a lockright in the front. With a martack.