I know that you mentioned that you replaced the wheel cylinders, but is your brake adjuster on the rear wheels working? If it won't turn freely, it's not. Every time you back up in a vehicle with drums, it adjusts the pads to where they need to be. If the adjuster won't turn easily, your rear pads probably aren't hitting the drum.
You can buy a kit for the drums to replace all the springs and whatnot. If this has never been done, it's probably time.
I doubt it's your front brakes. When the calipers go bad, it's doubtful that they'll both screw up the same way at the same time and if just one is acting up it'll pull to one side like a mother...
The proportioning valve is a potential problem, but I'd be more suspect of the master cylinder (I've had brand new ones that didn't work on more than one occasion).
All of my brake components are lifetime warranty at this point. If something acts up, I pull all of it, take it in and get new ones. Master cylinder, wheel cylinders, calipers, shoes, and pads. On the Jeep, I've never had a problem with any of the other components in over 300k miles. I'm running 33/12.50-15 Hankook MT tires on Centerline Hellcat (aluminum) wheels and I have to be careful when stopping fast to avoid locking up.
Upgrading the brakes is always a good idea, but it sounds like you have a component problem and that needs to be addressed before you start throwing upgrades at it that may mask the real problem. The most likely suspects are either the adjusters or the master cylinder. While proportioning valves and boosters can fail, it doesn't happen often. Stick with the basics and go from there.
“The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.” ― Robert M. Pirsig, Zen And The Art Of Motorcycle Maintenance: An Inquiry Into Values