Here's my $.02 (might only be worth that) I had a noise with similar behavior. The u-joints seemed perfectly fine when examined, but when I replaced them (on a mechanic's recommendation), the noise went away. You might try running the truck in 4wd without the rear driveshaft and see if it still makes the sound.

Last edited by BrokeAsAJoke; 09/09/06 03:21 AM.