We have the Tundra 5.7l and have the same experience. On level land we are able to hold the top gear, but any sort of incline and it will start hunting (yes, even in tow/haul mode). I just downshift into 5th or 4th gear and good to go, once the land levels back out, I drop it back in Drive.
2013 Jayco JayFlight 26RKS
2013 Ford F150 Ecoboost FX4
2003 Coleman Bayside / 2011 Jayco Jayfeather X20E -- RIP
2012 Tacoma Sport DCSB / 2013 Tundra SR5 - RIP