The spacer is the cause, your radius arm drop down brackets are only compensating for 4-6" of lift. You adding to that another two inches is changing the geometry. The radius arm drop brackets are designed to keep it close to stock geometry while adding lift, the pivot point on the bracket tries to keep the arm horizontal with the ground. The spring being the connecting points between the frame and the axle, the more additional lift you add to the axle side without compensating on the bracket to radius arm side will cause the spring to bow outward towards the front of the truck.
Take a spring out of a pen and put it between your thumb and index finger and try to compress it, watch it bow out and you have the same basic effect. It will cause the truck to ride stiffer as well since the spring can't vertically compress with out also closing the larger gap on the front side of the spring between the coils at the same time.
Hope that makes sense, kind of hard to explain in type.
