This is something I've noticed since I got the truck. It only does it when you're starting out at the bottom of a hill. When about half way up the hill at 2000-2200 RPMs or 35mph in 3rd gear it will lock the converter dropping the RPMs down to 1500-1600. Then around 40mph it will go into OD and the RPMs are around 1000-1200. It doesn't downshift, it holds OD and gains speed while climbing but I don't think it's normal to go into OD so soon while going up a hill, plus it's got to be working harder at the lower RPMs.

When on a flat road it goes into OD at 45mph, doesn't even think about going in at 40mph (I've tried to make it shift) which is what I believe is normal.