That's a very long 'but'.
Seriously - you're replaceable. Your bosses know that otherwise they'd be bending over backwards to keep you with offers of a directorship, payrise and parties with free bars and latino hotties. If you left, they'd phone up a recruitment agency and get a good-enough replacement within a couple of months.
I'm not trying to disparage what you do and are capable of - it's just that there are others out there at the moment with more experience than you who even be willing to relocate for a job.
So, lose this notion that you're irreplaceable, and focus on what you bring to the company, and use that as a negotiation tool :-)
I don't have a strange idea that I'm irreplaceable, just that my job is a bit more important to the company than a lot of people seem to think. It's a bit discouraging really.
Still, I think I managed to do what you suggested, in that I highlighted a lot of the positive things that I bring to the company, so hopefully that should help.