Is it only here, in the South Texas region, that there is such a high disparity in gas vs diesel fuel prices?

I know, I know, we are not exactly comparing apples to apples here. There have always been differences. But this large? One can get reg gas around here for $1.90, but diesel is about $2.93..... well above a dollar difference.

In the colder months there is much higher demand for fuel-oil for heating, which puts a regular seasonal pressure on diesel prices (at least in the UK, not sure about Texas).

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm

when in doubt, look at the relative taxes on each? Road-use diesel is much more expensive than non-road use diesel (which is color dyed and illegal to use in road vehicles). Find that cost to see the tax load.

Some locales rolled back taxes on gasoline to give consumers a break, but did not reciprocate for diesel.

And perhaps some built-in padding to cover the imminent transition to cleaner diesel?

Diesel is running about $2.75ish in New Mexico, and gasoline about 50 cents cheaper. As Clett mentioned, there is a seasonal competition between diesel for cars and diesel for home heating. In addition, Katrina may have promted lock-in contracts at higher prices that your Texas refineries are now filling, causing some local shortage.

R2-E2, 2G Prius.
Highway/City/Husband/Wife MPG: 56.5, as of 12/2005, 26K miles

Jac Nasser, Ford President: "We are planning to launch a hybrid version of
this car [P2000] within this year [1998]. We will also make FCEV available in
2004."

The diesel refineries are likely using this time to switch over their refining processes to the federally mandated low sulpher diesel formulations. That both reduces domestic supply (increasing the demand for expensive imported fuels) and also means the new ULSD will be a bit more expensive, although prices should be stabilized by fall '06 when ULSD is uniform among all refiners and stations.

I had read somewhere the military-use fuels are color dyed too, for similar reasons. Don't know if that is actually true.

The other two explanations that you provide, also seem highly plausible.

The seasonal re-stocking of fuel-oil for heating may also have an impact too, although my understanding is that fuel-oil (kerosene) is actually a different fuel more akin to jet fuel. Again, I'm not 100% sure about this statement.

nr2 heating oil is basically nr2 diesel with less filtering; it is not kero-based. I believe they are actually different distillates, but with very fractional differences, though they are sometimes interchanged in the real world.

Jet fuel (JP4/5/8) can be used in diesels, and vice-versa, but don't let a pilot hear that at 30Kfeet. Aviation fuels have a lower flash point than diesel, lower water content, icing inhibitor additive, and burn hotter.

military diesel is not dyed IIRC, but commerical marine diesel in the US and Europe is. I've seen green, red, blue, purple.