I tend to lube the chassis/driveline every oil change, so it shouldn’t be lacking in that regard. It’s a very constant tone (similar to a whistle) if the speed isn’t changing, so my thought is that it will be either one of the bearings on the output side of the transmission, or the ring and pinion. I’ll take another look for driveshaft interference and double check the U-joints.
Another note I forgot to mention, the noise occurs regardless of outside temperature, or vehicle run time. It’s been doing it for the entire time I’ve owned the truck(a little over 2 years), but the volume and tone of the sound hasn’t changed in that time. The fuel level doesn’t seem to effect the sound at all.
I guess if I can’t find anything, I’ll pick up an IR thermometer and take it for a long drive at 50. Where there’s noise, there’s friction and heat.