Here's what I did. Arm yourself with the following data:
1.0 Jack the truck up so the rear axle is hanging, then measure the length from the upper shock mounting bracket to the lower shock mount.
2.0 Now measure the distance from the bump stop to the frame.
Subtract the bump stop dimension from the fully extended length. Make sure that the shock you pick, when fully collapsed, is less than this dimension, that way it will never fully collapse.
Use these two measurements to pick the correct shock at your local parts store -or-
Go to the Monroe website
http://www.monroe.com/catalog/catalog_default.aspand use their step by step process to pick a shock and then got to this page
http://www.monroe.com/catalog/miscApplications.asp and click on the link "Mounting and Length Sheets" which lists Monroe shock lengths. Check this against your actual dimensions.