I'm thinking that when spring rolls around I may scrape off all the undercoating and treat any rust that might be underneath. Then I'll paint it all and re-undercoat with something. There are only a few areas where it looks like there might be some rust beneath the undercoating. Oddly though, the coating is flaking off part of the floor pan and no rust has formed there yet.
The plus is, in theory, the only thing I should have to worry about after the transmission is keeping the underside as nice as it is now for years to come. This Eagle will see snow and salt, so I want to make sure it's protected. Until spring gets here I'll keep driving my Jeep primarily, washing the Eagle every week to keep salt off. After I repaint the underside, I'll take it and get the internal panels/doors/etc professionally oiled.