Do you live in Milton Keynes?
I used to run Dunlop AT20s years ago and remember having funny wear patterns. Increased the pressures by about 10% on OEM and found that to be better in terms of handling but still had strange wear.
Moved on to some "proper" AT tyres (all 4 to Hankook Dynapros RT10s in my case) and any issues have gone - all 4 tyres have worn evenly (side-to-side), with the front pair worn 1mm more than the rear pair - but negligible at a glance. I run at 38psi all round and increase to 40psi if I'm planning a long M'way run.
My caster correction has been seized since day 1 and never been adjusted. Every 3-4 years I take it in for an alignment check, or after any work in that might impact the steering geometry.
If the 120 is similar to the 90 series, then there is nothing you can adjust at the rear, only the fronts.