Scott, you are right about the back-lash, that is the amount of slack between the gears. If they are too tight it puts a strain on the bearings because they are being pushed, too loose and it's hard on the teeth of the gear because you're not using the full face of the tooth.
Depending on how much you are going to use the pulley, you could do this job yourself. I'd start with all the shims they send you, and remove one by one til you only have about 2-3 degrees of rotation on the pulley with engine off and PTO engaged.