I'm going to guess that the independent mechanic replaced the "flex disk" which is essentially a rubber piece in the driveshaft line that absorbs some of the drive train shock. Did they give you the old part back?
Brownish transmission fluid and a light coating of metal shavings on the bottom of the transmission pan are not all that uncommon, especially if the pan has never been off (nor the fluid changed) in 74K miles. By "metal shavings" I mean almost a powder consistency. And maybe flakes.
I've seen and heard enough mis-diagnoses to think that I would get two and three opinions before any kind of major work on a car such as yours.
If you want an opinion over the Internet and won't hold me to it (
) I'd say in this case the dealer is right. And I would consider asking the dealer to take a look at the flex disk to make sure a good part was put in as a replacement.
And, I'm speaking not from specific experience with your car but from a basic transmission maintenance experience.