the plugs are in the drive unit (transmission)
80w90 gear oil.
small plug is level check, large plug is fill plug.

caster measurement... not really a spec for this, i generally shim them until it feels right. some of the newar ones have adjustment bolts/holes and also a nice rubber block that can wear out but still remain intact. when adjusting you have to make sure the drive tire still has enough pressure/grip to stop the truck both loaded and unloaded.