I was underneath my '02 F250 the other day looking at something unrelated and noticed what looks like a torn dust boot on the driver-side lower ball joint. There's a lot of "rust dust" around this area and it seems to be coming from the hole in the dust boot.
The truck has 138K miles. I don't know if the ball joints are original or not - I've had the truck for the last 10K miles. The truck drives fine. No unusual noises. The other three ball joints look OK. I tried the prybar-under-the-wheel test and didn't notice any movement.
I'm leaning toward replacing all four ball joints but wanted to get some expert advice first.
Here's a pic. The black smudges on the right side are from me. Thanks in advance for the help.