I can't find it documented anywhere but with the correct spring
height and no shim the axle should be at neutral caster.
Don't know what truck your asking about Jr. but the 3100 spec is
1 1/2° to 2° pos. caster. Your original shims were probably 2°
as the same shim was used for the 3600 also. And yes, you do
need them under both springs. I would think that if you're
missing the shims that you are having a problem with your truck
wondering and that you're having to chase the wheel to keep it
in a straight line?
Shims of various degrees are available at most shops that do
truck alignments.
DG
