Shock question?

[knobby]

I recently purchased a 2003 F250 6.0l psd from a dealer and it is lifted, they said it was 4" and me and all my buddies say 6", anyhow i see a very little bit of seepage from the left from shock and want to know which ones i will need going to replace all 4 but how do i know what shocks to get without taking the old front and rear ones off first then measuring them. Any help would be great