Stock shackles for 58-7/80 are 2-3/4" center to center.
A lift shackle is rated in how much more distance is between t hose two points. So a shackle advertised as a 3" lift shackle would be 3" plus the original 2-3/4".
HOWEVER... a 3" lift shackle will not raise your vehicle height up by 3".
One end of the spring is fixed, and the other is hooked up to a shackle. The axle is in the middle. Any increase in shackle length only changes the height at the spring by half that amount. So a 3" lift shackle only increases the vehicle height at the axle by 1.5". A lift shackle of 1.5" only raises vehicle height by 3/4". A 2/5" lift shackle raises vehicle height by 1.25". You can use this information to "fine tune" your ride height.
There is a point of diminishing returns. At some point, too much shackle increase will stop lifting the vehicle, and at some point can reverse arch the springs. A lot of hot rodders jacking up their cars found that out, to their dismay. This usually oinly happens with excessive shackle length, like a 6" or even 8" lift shackle.
Speaking of ride height, I like a little more lift in the rear than in the front. This way I know the rear will never sag when it's loaded up with passengers, cargo or a trailer. The drawback is you may have to readjust the caster angle. Mine is off on my FJ40, and I'm working to correct that.
So my shackles in the pic are only providing 1.25-1.50" of lift? And if I were to replace them, I would order a 1.5" lift shackle - correct?? Forgive me if you answered that somewhere in your original post Brian. Thanks.