Just got back from lunch. While I was out I jumped on the freeway and set the cruise (just me, no other cargo and no trailer). Every time I would hit the tow/haul button the trans would downshift to 4th, then upshift to 5th and hold 5th. It would never shift up to 6th. Deactivate tow/haul and boom it would shift right up into 6th.
Maybe it's just my truck or the conditions, don't know. But my truck has never shifted into 6th with the tow/haul button engaged...ymmv :)
I would go for the 4.10s to the other poster...it adds so much capacity and for $125 option it seems like a no brain-er in HD truck...