First off, what kind of wear does the slip yoke have? It can make that kind of noise. Can you get any lateral movement through the joint? Also, when you reassembled the two halves, did you make sure the joints were in phase? Another possibility is, you could have an Eaton locker for a rear diff. They make noise, especially around corners. Make sure your wheel bearings are tight and getting gear lube. May be your spiders, but if it's a locker, it won't have them. An Eaton has a removable "pumpkin" and a rear inspection plate. Over the years, that truck could have about anything under it.
Good luck and keep us posted on what you find,
Scott
