Your 2003 GMC Yukon XL is rated to handle carrying a 62# SOT kayak using the Yakima Mako saddles on the front factory rack crossbar and Yakima Hully Rollers on the rear factory rack crossbar. Just remember that you'll need to use a set of 4 Yakima universal Mighty Mounts with the pair of Mako Saddles AND another set of 4 Yakima universal Mighty Mounts with the pair of HullyRollers, so thats 2 four-packs in all of the yakima Universal MightyMounts that you will need. (or you can now choose the Yakima Mako Aero Saddles
which come with universal factory rack adapters included, in which case you'll only need one set of 4 Mighty Mounts to use with the Hully Rollers). Both the Mako Saddles and Hully Rollers each come with a bow-stern tie down strap also so you can secure the front and rear of the kayak during transit. This set up will make it very easy for you to load your sit-on-top kayak and ensure the boat is secured safely during travel even at highway speeds.
http://www.orsracksdirect.com/yakima-universal-mightymounts-clamps.html
http://www.orsracksdirect.com/yakima-hullyrollers-kayak-rack-8004035.html
http://www.orsracksdirect.com/yakima-mako-saddles-kayak-rack-8004037.html
http://www.orsracksdirect.com/yakima-mako-aero-kayak-rack-8004044.html