The upper ball joints are not load bearing, as are the lowers. I know you think you are getting conflicting advice, but you really aren't. Your mechanic says he's only seen 3 failures in 42 years, but I bet he's never seen one fail at all on a 96 Chevy Caprice.

My point is not his ignorance, it's his lack of exposure. I've been working on cars since 1981, and I've never had first hand experience with LBJ failure on 3rd gens. But I KNOW they fail, and at 193K it's a high priority for me already, even though I plan to SAS my truck eventually. No one on this forum has anything to gain by telling you to change them both. They are just sharing what they've learned.

One more thing, as a shop owner if a vehicle came to my shop and one LBJ showed visible movement, I would only perform the repair if they replaced BOTH SIDES. In the automotive world there is a saying, "if one is gone, the other is going".

Do the right thing Mookie.