It is a new truck but my wife would like a smoother ride when we are riding around town empty.

What would provide a smoother ride differnent shocks or air bags? If so, which brands?

Keep in mind, I will still need to pull a 9,000 pound travel trailer.

shocks will give you what you are looking for the airbags are more for helping when you have a load if you don't have a load they will stiffen up the rear of the truck my buddy keeps his set a 20psi and seems to help so maybe they do but i haven't riden in his truck since we put them in so i can't be positive about what he is saying is happening

i have different shocks and air bags in mine. For some reason around 30 psi in the bags and it rides the smoothest. play around with the air pressures.

i would add shocks, air bags will stiffen the ride but are smoother than AAL's if your needing help in the rear when having added weight

With My bags @ 20psi it rides alot smoother

I'm wanting to smooth out the ride on my truck also. When you add airbags do they replace the rear shocks? The bags look like they are installed between the springs and the axle is this right?

no they don't replace the rear shocks they replace the rear bump stops they mount between the frame and the axle on that flat pad were the bump stops would hit if you bottomed out

I spoke to a person at a local, respectable, 4x4 shop yesterday. His suggestion was Rancho shocks with an adjustable knob (sorry - don't remember the number) on the front and air bags on the back.

I am wondering if adding the 300 lbs of a future truck cap would settle it down?

Do shocks all around. IMO, don't just do the front. I just installed the Rancho 9000's (most likely what the 4x4 shop told you to get) over the weekend on my brother in law's Ferd. They're pretty nice, pretty stiff, but you can adjust them. I'm really happy with my Bilstein 5100's I just put on a month ago. Putting air bags on either tonight or tomorrow. I'll let you know how it changes things. I'm towing 12,000 lb toy hauler. I love the ride as is with the new shocks/leveling kit, but now that I'm level, I sag too much in the rear so that's why I'm doing the bags.
Being that you're stock height, start with shocks and see how the Mrs. likes that before you got to bags. Canopy will help a bit.

I spoke to a person at a local, respectable, 4x4 shop yesterday. His suggestion was Rancho shocks with an adjustable knob (sorry - don't remember the number) on the front and air bags on the back.

I am wondering if adding the 300 lbs of a future truck cap would settle it down?

Rancho 9000s.

the cap will help. get the shocks. rancho or bilstein's