When I needed new shocks after my lift (and f-250 mounts) I figured out the length using math AND by measuring.
You can use math to figure out the new extended and compressed length BUT that doesn't really take into account the new travel (assuming you went longer radius arms etc).
To measure it I took out the shock on the driver side and lifted the front left of the truck by the frame. I used a tall jack and jack stand. I then took off the tire and put my weight on the end of the axle. That was around the max droop. To get max stuff I disconnected the shock and drove it diagonally through a ditch to get the suspension to stuff on that corner. I know I didn't get max stuff (didn't hit the bump stop or compress the coil)so I subtracted a little more to get my compressed length.
I also checked my numbers by figuring out how much lift I got and then added to that length the difference in height of my shock tower (mine was raised).
Once I had rough numbers I looked through the Rancho catalog (@ gorancho.com, the product catalog page) to find a shock with an extended length of at least what I came up with and a compressed length that was no more than what I came up with (with the right mounts) I tried to go a little less in compressed and a little more with extended in case my number were slightly off.
~Mark
