The shortening service I offer is for the front YFZ shocks. Unfortunately the stock YFZ rear shock uses a completely different lower mount, and will not work on the Banshee with stock linkage.
If you are looking for a revalve, your stock rear shock is the way to go. I only charge $85, plus a $40 seal head and $5 shipping to do your stock rear shock.
Total, out the door for your stock rear, revalved, you'd only be looking at $130. Unless you need a new spring; that option is by far, the most budget friendly way to go. I also stock Eibach rear springs in just about any spring rate you need, for $104 installed with any other service.