Cupping is a problem that most of us will eventually experience over time. Short of buying the exact same tire for the rest of our lives.
Too often this is simply the result of failing to maintain proper tire pressure. However, this is far from a complete answer.
Cupping is a phenomena that is absolutely normal. Excessive cupping or excessive wear on one side of the tire as compared to the other is not.
There are at least eight causes of cupping and/or uneven wear:
1. Most roads are banked away from the center or crowned as most like to call it in my area.
2. Your tires 'scuff' when you force a speed change with them. The rear tires scuffs when you accelerate and when you brake.
3. Alignment.
4. Carrying an unevenly divided load.
5. Defective or worn shocks.
6. Excessive front brake.
7. Tires containing large stand-alone tread blocks
8. Air pressure
Mud tires are notorious for cupping. Mainly due to number 7. Some guys call it (Tread Squirm) MUD tires require far more rotations and are more sensitive to air pressure levels.
With all of that said... I don't know what your problem is, but IMHO, it sounds like a combination of #5 and/or #7 and #8.
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98. 5' 24v 2500 Auto/3. 54 4x4 SB QC. Everything but leather. PIAA 1200's, AMSOIL dual filter relocation system, Smittybuilt Stainless Steel Nerfs, Rhino Liner(Junk), K&N air filter(For Sale)15 year AMSOIL dealer. Time permitting, Soon to include Ultra-lite Pyro,0-50lbs boost, Trans temp in the pillar. Rancho 9000's with in cab adjustment.