Great thread to start.
As far as I know, the furthest west that Yuengling is available commercially is Memphis. Although the closest location that you can buy it is the first Alabama exit on I-10 East (Grand Bay, AL). Most gas stations and stores off of this exit have Yuengling Traditional. One grocery store about 1.5 miles south of the interstate has Traditional, Light, and Black & Tan (which is fantastic BTW).
Yuengling has two breweries. The original one in Pottsville, PA and a relatively new one in Tampa. Due to their limited brewing capacity and the popularity of the beer, they are stretched pretty thin and are limited to mostly the Eastern Time Zone.
My suggestion, anytime you're heading east past Mississippi make sure you buy a couple of cases on your way home. Also, if you have family that may be heading out to Florida for vacation, give them some money to pick some up for you on their return trip. My sister offered to do this for me a while back.
The Yuengling FAQ page linked below actually does a pretty decent job of explaining the beer's limited availability.
http://www.yuengling.com/faq/