Me, my dad, and my girlfriend went from near St. Louis to Virginia Beach to pic up my brothers wrecked car and bring it back home. We travelled I-64 all the way there. The truck is a '96 Z71, 5.7, 4L60E, 3.73 gears, factory transmission and engine oil coolers pulling a 16' utility trailer with NO brakes. Our gross weight was 11,520 pounds. The truck did real well through the mountains. I set my cruise on 70 and left it in OD for most of the trip and it would downshift on the steep grades but never fell under 60 MPH, and never downshifted on small grades or flat ground. I kept it pulled down in third gear where I-64 and I-77 run together tough. It was cool to visit my bro, he is in th Navy and lives in Va Beach. He works on F18's and took us on base so we could see his shop where he works and we got to walk out on the air field where all the jets park at. It was cool.
And the wannabe tow rig:
And the wannabe tow rig:
I really like my bird even if it is a ford.... 
Towing in OD is a no no with almost any tow vehicle. Its not just a matter of whether or not it can maintain speed in the gear, OD is weaker and you WILL kill it towing in OD. My understanding is that even a lot of the HD manuals are not meant to tow in OD.

