The valving refers to how much motion/force the shock will dampen...
If the shock is too stiff, the tire hitting a big bump pushes the truck up too, essentially the shock doesn't absorb the movement, it transfers it...and you feel a big bump. When cornering, you have little lean, and quick recovery after lane changes, etc.
If the shock is too soft, the tire can hit the bump and come flying up into the wheel arch, whacking your bumpstops, etc...going back and hitting the ground again, bouncng up again, etc....and when cornering, you will be wishy washy, take a while to recover from lane changes, etc...as you are essentially bouncing around on your springs.
When the shock valving is just right, the impacts are absorbed, and you float over bumps, the tires going up and down following the ground's contours perfectly w/o bouncing, and the truck staying level and unperturbed.
Higher rate numbers are typically stiffer, lower are typically softer...and there's a compression and a extension stroke, so the piston is damped going up, and going down, at different rates (Think of one rate for controlling the bounce up, and the other for getting the tire back onto the ground again, etc...)
The right rates are determined by both comfort vs control preferences (your's), as stiffer handles better, but softer rides nicer...and every one likes a different balance between confort/performance....AND the weight of your rig (Front and back...) AND the speeds and types of terrain you are prioritizing the rig for.
Did you take the rig to a truck scale, and get the weight at all 4 corners yet?
If you want a short cut...I like the Bilstein's from SLR...the stock, 10% and 20% stiffer shock choices are pretty good....and there's also the adjustable Rancho's...you play with the settings until you adjust them to your liking. People really like both the Bilsteins and the Rancho's around here. (10% stiffer good for normal weight, 20% if adding bumpers, carriers, winches rails, armor plate, etc...)
If you might lift in the future, get one's for a 3" lift now, they'll work with the pre-lift height too.