Tire, Frame, Steering Assembly??

Your original post with the what the heck happened part made it sound like you were surprised. 300k mi. it owes you nothing and if it were a horse they would have shot it.

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"Education is the ability to listen to almost anything without losing your temper or your self confidence"
Robert Frost

Good point!

Not upset that it needed repairs, surprised at the instantaneous way it happened. I honestly thought the frame broke from the look of the tire and how the truck was sitting. By the time the mechanic said ball joints, I was just happy it was fixable.

I have to say, those Toyota's sure take a beating and keep on running.

I don't think your ball joint broke. I think it separated! The ball and socket were worn enough to allow the separation of the two.
The traditional way to check lower ball joints is to place a jack under the control arm to take the load and raise the tire an inch or 2. Then try raising the tire with a pry bar and look for movement in the ball joint. The ball joint could show no movement and be worn dangerously if the wear space is filled with dirt and rust.
Measuring from the top of the stud to the bottom of the ball joint is recommended but can only be used if you know what it was before wear.