Questions on towing

[orygunf150]

Had my first experience towing with the f150 w/ gryphon. We bought a toyhauler, well to haul our toys of course. It's only 20' total in length and only weighs 4500 lbs. We towed it home about 180 miles in 100 degree down
I5 in tow mode (level 2) encountering a few 6 % grades. This is an 05 f150
4x4 with the 5.4, 3.73 gears and 235/75/17 tires. Being a little concerned about the temp I monitered engine coolant temp as wall as tranny temp. On flat roads coolant temp ran about 190, tranny about 175. At the top of the hills I was about 202 and 195. I think those temps are not bad towing in 100 degree weather, opinions? At what point would you become concerned with tranny temps? On to the real issue, driving on the flat areas at about 65 mph in overdrive the torque converter seemed to hunt between lock and unlock (I was also monitoring comm gear) so I locked out the overdrive towing locked up in 3rd. FWIW the tranny temps didnt seem to be affected by the hunting or the gear change. I was a little surprised that it wouldnt hold 4th gear locked up since this isnt a lot of weight and the fact that I have 3.73s and smaller tires. I recorded the following at 65mph:
4th gear locked up: 1900 rpms
4th gear unlocked: 2500 rpms
3rd gear locked up: 2700 rpms
This brings up the age old question, should I tow in overdrive or not? Also, should I customize my tow mode, or have a custom program made?
Opine at will............Mark