Just wanted to update and let everyone know I fixed the problem ...
I kept thinking this had to have something to do with all the 4x4 driving I had done in the snow just the week before the sound started. I drove around with a friend who is a diesel mechanic by trade. Just as soon as the sound started he said he thought it was coming from the front hubs. He said to keep driving and accelerate up to speed. When I got up to about 50 mph he flipped the switch on the dash from 2-wheel to 4-wheel high and immediately back to 2-wheel. The sound went away for about a minute then came back. We did that little switcheroo about 3 more times and it finally took. Evidently the front hubs were not completely disengaging. I asked him if he had any ideas why and he said probably age, lack of lubrication and the fact I hadn't used them in years. I'm going in for an alignment and rotate/balance in the next couple of weeks and I'm going to get everything up there lubed up at the same time. Ball joints, U-joints, hubs, you name it. Anything they can squirt grease on/in and definitely anything that has a zerk on it.