So, I am wondering, with only two years of webdev under my belt, am I marketable out in the rest of the world? If so, would I have to take the pay cut to be a Jr. Java programmer?
I'd like to think there'd be a fast-track for me to become a Sr. Java developer, maybe even a Sr. Java Architect someday, because of transferable skills I have from the Enterprise Mainframe world. But are there any critical technologies I should work with before making a leap? Or is it a matter of acquiring more time on the webdev side (as perversely retarded as it may be in my current place of employ)?
If your web development skills, especially those for the client-side technologies, are current I would think you have a leg up -- people with skills on both sides of the HTTP divide seem to be in demand. Though I can't necessarily speak to demand in your area.
Do not, I repeat do not, bill yourself as a Java developer. Do not even think of yourself in those terms. Kiss of death. No one wants someone who is focused on a single technology.
At the risk of sounding cynical, I've seen too many Java-oriented "designers" and "architects" who lacked anything like your depth of experience, so don't sell yourself short. If you're keen to work in Java-land, maybe you could pursue some of the higher certifications with the aim of pushing your way into these kind of roles, where your broader experience would be both valuable and (hopefully) valued? But like Bear says, don't be too myopic about it - keep an eye out for opportunities to extend your skills beyond "Java development" and watch out for the Next Big Thing where you might be able to get in ahead of the crowd (e.g. "big data" seems the current NBT). Good luck!