Was working on a very early build '97 F150 2wd today, and lo and behold! There once was a boss on the frame on both sides of that rear control arm mount for eccentric adjusters like on the upper arms. This truck had the eccentric bolts and washers and even marks by degree on the washers.
I guess they decided that the fifty cents of welding the bosses on was too much along with the eccentrics. So all the newer trucks end up with the mobile left control arms.
Nice one.

 

I wonder if those parts are still available? would be hard to tack a couple guides on.