Yep, I didn't even think of that. Thanks for the help guys. So, how would I check for u-joint wear, other than obvious things such as grabbing the driveshaft and seeing if there's play, or seeing if it's lubed at all? I'm gonna do a search right now, but any advice is appreciated.

One thing though: There really isn't any vibration at all, at any speed. I mean, things vibrate and shake somewhat at speeds over 60, but no more than you'd expect on an 18 year old truck with 187k.

The "humming" does seem to be worse when coasting. If I'm coasting down hill, it sounds as loud as the engine does at 3500 rpm, and very similar in tone, but if I put it in gear and give it some gas, the humming quiets down slightly... not much, but enough to notice it.