I retired from the Tire Industry and prior to that spent 15 years as a factory rep for a manufacture of balancing equipment.
Most people, including those in the tire shops, don't understand what makes a tire ride smooth. If they don't see any run-out when turning the tire/wheel on the vehicle or balancer they think all is OK and just "spin er up", hammer some weights on, spin again and if everything "reads zero" its got to be OK.
First, think of a tire as an infinite number of springs that hold the wheel up off the ground (not exactly but close enough for this exercise). Some of those "springs" are softer than others and some are stiffer than others. This results in a tire, while looking round when not loaded, running down the road as if it had "corners on it" (square).
There's a machine out there that measures this "Loaded Runout" as well as balancing. Many Les Schwab stores have them, Discount Tire---Doubtful. The entry level for this machine is around $10K while a ordinary computer balancer is about half that.
My guess is that he OP has an issue due to a tire that doesn't roll round and could also be improperly balanced.
Tires are a funny item. Manufactures start out by designing the perfect tire. After manufacturing costs, selling price and profits are then factored in, they start removing some of the features or materials that made the tire "perfect". The lower down the "Totem Pole of Price" you go, the greater the chance of having ride disturbance, longevity, or durability (resistance to road hazards) issues.
FWIW, I too own a Titan. Last set of tires ran $800 and when I'm driving over 60 mph (sometimes a lot more than that) they ride like a dream. I expect to see close to 100k for mileage as I rotate every 5k like clockwork and keep them properly inflated.
If Discount can't (or won't) fix the problem, check out this website to locate one of the "Road Force Measuring" wheel balancers near you. (my local Nissan Dealer has one, yours may also)
http://www.gsp9700.com/search/findgsp9700.cfm The cost of "Match Mounting"can be less than stepping up to a more expensive tire. This is where they take loaded run-out readings on the tire and wheel, separately, then match the "low spot" of the tire to the "high spot" of the wheel, then balance the assembly. A good shop will even match the tire with the greatest loaded run-out to the wheel on your vehicle that has the greatest run-out, thus cancelling out more vibration inducing issues.
Lastly, tires that have this done to them often last far longer when driven exclusively on the freeway.