I sent all 4 of mine to Fox to have them revalved. Its $125 a piece. I have an extra set of fronts and I bought an extra rear and swapped those 1 at a time so I could still drive the truck.
Rears are a piece of cake to swap, only 2 bolts. Fronts are a little more tedious, unbolting the UCA is the easiest, then the whole knuckle drops down and you can take it out.
There is basically no difference between Scab and Screw shock, an additional shim is used inside for the Screws.
Fox recommends rebuilds every 50k miles for everyday street use, mine lasted til about 70k. Once they start of leak the bottoms will accumulate fluid on them, turning them black, very easy to see this.