There are some clutches in the converter that unlocked are allowed to slip whereas locked the converter is 1:1. If its unlocked while driving itís like having your foot part way down on the clutch pedal and letting it slip a little bit. While slipping it makes for softer gear changes and allows smoother power delivery, helps the motor stay up in the RPM band, and all of that generates large amounts of heat which is bad for the tranny. Now when youíre in 4th and unlocked and pulling a hill you are essentially losing power into the converter that could be going into the tires. I would say you want that sucker to lock and stay locked as soon as possible under most circumstances.
2008 F350, Crew Cab, FX4, SRW,
job 3, Long Bed.