Measure your suspension travel...
Jack your truck up and let the suspension droop. Measure the distance between the lower and upper shock mounts... (this will be your extended length.) Then, find a way to compress your suspension or make a very educated guess as to how far your suspension compresses under load or during your suspension's cycling. (This will be your compressed length.)
Find a shock that matches your extended and compressed lengths, and you'll be good to go.
Jack your truck up and let the suspension droop. Measure the distance between the lower and upper shock mounts... (this will be your extended length.) Then, find a way to compress your suspension or make a very educated guess as to how far your suspension compresses under load or during your suspension's cycling. (This will be your compressed length.)
Find a shock that matches your extended and compressed lengths, and you'll be good to go.