What brand/Model/size (Hope they're not Goodyear cause they haven't had a good year for a while) Does the steering wheel have a rapid left to right to left.....feeling, or more straight up the steering column. It can be very helpful to have a driver & observer(a front end tech would be nice) in another vehicle that you fallow them closely and gradually increase speed into the shitzone. Sometimes closely observing components as problem develops is helpful, To much toe in can do it. also have visually inspected while driving parallel to the left and right side of you.
Still wondering about tire info. To completely rule out tires, driveing on another set w/ a completely different type of tread pattern with the same result would completely exonerate tires as cause. Knobby off road tread patterns w/tread rows interrupted by crosscuts are inherently problematic submitted to paved surfaces at high speeds. Parrelli low profs. smooth at 140 mph all day, but 10 mph up a muddy hill aint happening all day, if ever. Dual purpose tread designs allows multi-uses but none of them well. Some hard core specific purpose off road tires imprint a disclaimer to expect shitty paved road performance. So barring exotic treading design, the roadforce balancing of two sets, and the type of vibration (rapid L-R-L-R) points to suspension or steering assembly in their relationship to tracking . Toe in or out alignment and degree"s of or+/ - camber angle.looking straight at front of truck,tops of tires lean towards or away from each other (like Knock- Kneed/bull legged) and tires direction angle toe out or in (If skiing,out causing splits & in is snowplowing effect) out tends to search or drift, toe in tends to cause your type of wheel vibration. How many miles of the 500 were yours? Steering alignment came that way from factory or damaged/ tweeked prior to you accepting delivery.
Apply this concept to what you've experienced so far. Unless you noticed the problem prior to and noted that your acceptance in takeing delivery and assumption of legal possession is contingent upon rectification of exsisting streering dysfunction in a timely manner and to your satisfaction otherwise your legal acceptance becomes null & void, legal possession shall revert to seller and purchase contract is to be considered null & void the result of sellers failure to satisfactorily perform, The longer you keep possession & continue use of vehicle,the less your ability to demand the replacement of the vehicle based on a Known defective state of the vehicle or effectively recind the contract. So, they brake out the stall tactics". Examples:Those darn tires! keep it till next week cause our best tire guy will be back from vacation or it well be slower so we can give more attention. Tires still screwing up? bring it back next week and we'll do a state of the art,special super duper never fails rebalanced on em.take care of this once and for all. Still not right? bring it in next week and we'll order new tires & rims as yours must be defective. It's rare but it does happen. Still not right? The dog & pony show continues and the weeks & miles keep going by. If you had noticed it on the drive home , turned around,re parked it back on the lot and gave them the keys refusing delivery until its right or exchanged, you might be driving another truck and some other poor guy ended up w/ this one. Dog & pony shows included with standard equipment. Once you sign the contract, the only thing that is separating your salesman from his commission on the sale is your taking possession so they tend to be "the Showmaster" and maintains a good working rapport w/the service department guys. Even worse, a return causing commission to be deducted from his next check. Take it to an experienced alingement shop and have it inspected and test driven. Tell them it came this way, Try not to choke on your gum when tires & rims are the very first items ruled out as possible cause. Documentation implying cause was obviously unrelated to tires & rims. Then go see the dealers Gen, manager and tell he how much you enjoyed their D & P show and after a second opinion, you thought it also might be considered a acceptable example of conspiracy to commit fraud for the purpose of monetary gain by the dealership and a certain individual(in many states persons paid commission only are considered Independent contractors and can be sued individually) your wondering if he had any thoughts regarding the subject. Good luck! It's painful to see years of customer loyalty and trust treated with complete disregard. Starting w/ a 62 E-100 Ive purchased 11 fords for personal &business use. Not from any problems I've had, but from what I've head of & seen others experince, I have no intention of rounding off the dozen.