What about the double down thing? You've technically trippled down now, right?
I don't remember exactly what I said in your previous thread, but it was probably something like stick with what you have if you like it. A newer truck is going to cost you a LOT more than $3500 a year... And you now have a NEW trans, which was probably a good move, even if you did pay more than you really wanted.
Switching to a newer used vehicle is always a gamble, since you have no idea how the previous owner treated it. In my mind, you simply got unlucky with a rebuilt transmission. Move on.
Side note, it's odd that you had a sudden pump failure... They usually don't just fail without warning. The seals can go out, causing fluid to leak out and eventually running the trans dry or low and killing the pump, but even with the seal leaking, you get symptoms like hard shifting and noise before the pump fails. I'd be curious to know exactly what happened to cause the pump to fail... Transmission pumps are not exactly complicated...
Hopefully the transmission shop flushed the cooler loop well when they installed the new trans. I think you've got plenty of life left in the truck, and if it's getting the job done, I'd consider the new trans an investment in longevity. Adding the cooler was a giant step in the right direction.
If you do want to consider something newer, I'd do yourself a big favor and look into something with a turbo, and with at least a 6 speed trans. 4 speeds is fine for flatlands, but here in Colorado, you're really hurting for gear selection, especially with a non-turbo motor. I'm not a Ford guy, but even I'd consider an Ecoboost. I think having the turbo and a 6+ speed trans would make for much more relaxed towing through the mountains.
The idea of going to a 3/4 ton is only good if you go new enough to get a 6 speed. My 6.0L 2500 with a 4L80E screams it's brains out in 2nd and 3rd when I tow with it through the mountains. No different than your 5.3L I'm guessing. The 6-speed trucks I've been in do the same towing with much less fuss. A truck with a turbo will be even better, since it'll make up for the loss of power at altitude that a N/A engine has.
You might keep an eye out for a Ram 1500 with the ecodiesel. The Ford Ecoboost is another option. A used Colorado Duramax is fairly pricey still, but they might come down in a few years. If you're not afraid of Nissan's, the Titan diesel is also an option. That keeps you in a half ton, more or less but gets you a truck better suited for towing in the mountains than your 'burb.