treebound 

You can put some higher pressure road tires onto your MTB to improve the feel and speed on the street.

Or look at these local options, not necessarily faster or better, just ones to consider.
Fit is most important, functional quality is probably next.

http://jacksonville.craigslist.org/bik/803262618.html

Can't really tell what these are, a GT and maybe a Raleigh**********
http://jacksonville.craigslist.org/bik/799913346.html

Try an offer of $150 for this one, a 1987 model according to the seller:
http://jacksonville.craigslist.org/bik/797873539.html

Sometimes bikes listed only by their color are worth checking out, sometimes not, often not, but you never really know until you do:
http://jacksonville.craigslist.org/bik/790969263.html

Maybe the Trek with some road tires, depending on what it actually is and if it fits and if the condition is okay:
http://jacksonville.craigslist.org/bik/790309408.html

A possible, offer less if it fits and is okay:
http://jacksonville.craigslist.org/bik/782326969.html

A strong possibility, slip on some road tires, turn the handlebar stem around to the correct position, maybe a set of handlebars for more road comfort:
http://jacksonville.craigslist.org/bik/780207646.html


Offer less if you can live with the color:
http://jacksonville.craigslist.org/bik/778248311.html

Nothing else on your local CL's first three pages really look like anything I'd even look at for the money. Also looks like you've got quite a few stolen bike reports and quite a few higher priced fixie conversions, so be aware of what you're buying and who you're competing against.

I'll just go back to what I said, try a set of road tires on your current bike first and see if that improves things for you enough.