I'm assuming when you had the belt off and engine off you turned each pulley by hand to see if you could feel any roughness when you rotated them (bad bearing/bushing). If not, I would definitely do this next. The other piece that you added about the noise being more pronounced when cold and when there's an electrical load causes me to lean towards the alternator. Not sure if you've ever replaced it, but it might be worth checking it and the condition of your battery. When you first start the truck there is a draw on the battery from the starter. This would trigger more output from the alternator as would the other situations, e.g. headlights, windows, etc. While whining can come from bearings/bushings, it can also come from an alternator that has a faulty bridge rectifier (AC to DC convertor). I've also seen where a bad battery will place greater load on the alternator, even though the battery has enough energy to initially start the vehicle. That's why I mentioned you should check it along with your alternator.
If the charging system checks out okay, then my next point of interest would be the power steering pump. If it is low on fluid or just wearing out, it too can produce a whine noise that is more pronounced when cold - until the fluid warms up.
Hope this helps and will be interested in knowing what you find.