65 psi in the rear tires is still pretty high. The rear of an empty pickup is a lot lighter than the front, and you are only running 50 psi in the front. I'm not saying the only issue is the tire pressure, but I would at least recommend lowering the rear to maybe 45 psi just to try it out. The TPMS warning will come on, but it's just a dash light.
I would suspect the biggest ride quality improvement would come from the rear. As already stated in other posts check out offerings from Thuren. They have a soft rate rear coil that is advertised as being closer to a 1/2 ton truck, just keep in mind that you may need to do something to handle the weight when towing. Airbags are fairly cheap and easy to install, and can be adjusted to compensate for the weight. I have airbags on my Power Wagon for towing (much softer rear springs than a normal 2500). Thuren offers a bumpstop setup where you can go back and forth between normal stops when empty and a load control setup. Supposedly pretty easy to change the setups. The load control setup is the same concept as a Timbren. I'm sure they are a good setup but cost about double that of a basic airbag setup. And while they look quick and easy to change, it's not as easy as it is to adjust airbags.
Shocks may help with the ride quality some. Keep in mind that the coil springs are controls the ride, and the shocks just dampen the spring movement. Some people swear that new shocks made a huge improvement in ride quality. If the current shocks were completely worn out I'm sure new shocks would help.......just my personal experience is that I've never really noticed a huge difference in ride quality on the street with new shocks. On my 2500 I put Thuren front coils and their Fox 2.0 shocks on the front at the same time. Honestly didn't notice much difference in actual ride quality on normal paved roads (again, this is in a PW so softer factory front springs than a normal 2500). At a later time installed the Fox 2.0 on the rear. Maybe a small ride quality improvement after this (or maybe I'm just trying to justify the $400 cost....).