I can't speak for your truck or automatic transmission, but I tow my 12,000 trailer in both 5 and 6th gear on my truck. With the 68RFE auto the 5th gear is a single overdrive and the 6th gear is double overdrive. When in the mountains and towing I watch my tach, EGT's, Boost pressure, engine RPM's and transmission temps. Down shifting to 5th gear out of 6th gear if needed to maintain the speed limit.