You guys are gods to me!
I had a little time on my hands, so I heated the nipple, sprayed WD-40 in it then clamped it in a wooden vice. I put a 3/16 socket over it and banged it several times, as I pressed down and twisted tiny amounts.
I thought at first I had totally stripped it because the socket was spinning-- but it was coming out.
I replaced the nipples with some Tesos that didn't fit my Uberti Pocket Colt. They seem perfect... could that be right--are Uberti and Ruger nipples the same thread size? (I snugged them up with a litte anti-seize goop.)
That's a monkey off my back. Thanks, again.