Shift slippage when going into overdrive at about 45-50 mph, with light acceleration. Normal shifting with no slippage with moderate to hard acceleration.
This problem just recently started.

 

How many miles on the truck and transmission? You could try to adjust the tv cable a little bit to boost the pressure slightly and give slightly firmer shifts. The stock overdrive clutch pack is pretty inadequate. If it's getting to the end of its life it'll start to slip under normal/light driving because the line pressure is lower than during moderate/heavier driving.

If if it's worth it to you to have it rebuilt, sooner is better than later. The sooner you have it rebuilt the better the chances that less hard parts will be wrecked or damaged.