towing w/ work truck this weekend-need advice

Put it in Tow/Haul, Drive, and hit the throttle. With a 6,700# trailer it will pull it fine and if you need to go faster push on the throttle. That engine doesn't develop close to max power until well above 3k rpm. If you find it is "hunting" for gears pulling a hill put in "M" and select the lower gear until you get to the top and then back to "D"

Sounds like you are nerveous about running up the RPMs with your comment about not liking above 55mph. That engine will run all day long at 4,000 rpm if it needs to.