So I have more info on mine...when cold (32 deg today, idled for 15 min before driving off) and I had pretty significant drag. In neutral it would stop the truck on my driveway hill. Feels a lot like brake drag, however after a drive, all brakes were within a few degrees. I did notice I needed quite a bit more throttle to maintain 60mph when cold, was in 5th with ECO hubs to maintain speed. That cold gear, tranny, and engine oil likely being the source of that. After the truck warmed up, it no longer had the drag and braking effect and cruised at 60mph in 6th easily.
Cold fluids would slow a roll down a hill but I wouldn't think there would brake to zero unless the thicker (15w40) tranny oil somehow causes this locking effect?
It still could be brake drag that works itself out quickly though.