in free-stance no load, measure the distance between bumpstop and frame, then measure shock mount to shock mount, subtract lesser from the greater, that is your compressed length. any shock that has a longer compress length that that will bottom out the shock before full compress of the leafs.
the extended length - simply unmount one end of each shock then lift (somehow someway) the truck so rear wheels can dangle (no jack under rear diff, etc). now measure shock mount to shock mount. this is your ext. length. any shock with a lesser ext. length will bottom out before your leaf gets to full extension.
travel - ext. minus compress.
you will most likely never see full ext. or compression, but now is the time to get correct shock length to take full advantage of full leaf swing. you may find that you need to relocate the shock mounts to get the best shock to fit and work correctly.