With the vehicle on level ground; Measure from the top to the bottom mounting points. Subtract 4-5 inches and search for a shock with your measurement equaling full compression for the shock you intend to use. You should be able to locate a Rancho "shock guide" on the net. This will give you the greatest amount of travel without bottoming the shock out. Here's a link to my setup.
If your gonna wheel that truck...forget about meas. at static height, go somewhere and get the suspension crossed up,then meas. the shortest meas. and the longest then take those meas. to where you buy shocks and give them the spec's and they'll tell ya which ones to buy,or come here and tell us and we'll tell you which ones to buy