I ran the Toyo Open County AT's for two years on my Nissan Frontier. I just took them off.
The Good:
The tires wear amazingly well. I have a friend that has gotten 80,000 km out of a set on his Pajero (Montero). They are great highway tires; they make no noise and I always got great millage with them. I wanted one set of tires to run year around and we have snow and ice to contend with up here, they have great scyping resulting in amazing winter performance for what is essentially an all season tire. Their wet weather performance is also way above par.
The Bad:
As great as they are on road they suck in the mud. If the trails you like include mud then be ready to spin the heck out of these tires, otherwise they will pack up with mud and you'll be asking your friendly neighborhood Jeeper for a tug. I didn't have any traction or flatting issues with them on hard packed trails, but at the same time I was too afraid to air them down below 20psi since they only have a 2 ply side wall. I don't have much experience with them on rock, and didn't have them on the truck for my recent Moab trip, however I'm guessing that they would do well in Moab, their plentiful ridges would grab the slick rock and take you up about anything (assuming the sidewall held up).
Mine were 6 ply tires and I ended up getting a flat by pounding a rock into these tires driving up a fire road. I thought it was dumb luck, but my local shop showed me a 10 ply Toyo AT that still had the rock wedged in it. My guess is that the scyping that makes them such a great winter tire packs up with rocks which work their way through the tread.
The Ugly:
I don't know what kind of trail you explore so it is hard to pick a good tire for other people (let alone myself). I'm going to go out on a limb and guess that there isn't allot of mud were you are talking about going, but at the same time I'm guessing there isn't allot of snow or ice.
When you say 99% highway use the Toyo AT makes sense, but it really comes down to the other 1%. Will you be airing down allot? What kind of back roads will it be on? Mud? How cool do you want your truck to look? If I were just using my truck on the highway and on back roads (with some light trail duty mixed in) I would probably have stayed with the AT. My choice to tackle more aggressive trails, mixed in with the amount of mud I have to drive through to get to a decent trail made me switch to a more aggressive tire (Cepek FC-II). The FC gave me the safety of a three ply sidewall while still offering the winter traction characteristics of an AT tire. I know the FC won't be as good as the Toyo on the ice, but that is the sacrifice of a more aggressive tire. As for the millage... I miss my Toyo's
Good luck on your decision.
Maurie