I do respect the knowledge of others here, and you should, too. Still, 78,000 miles is not really that unusual. Unless your truck has seen severe duty, there's a good chance the fluid isn't too bad. And, as '94 Van' says, you CAN tell by looking at it.

Certainly it would have been better to have changed the fluid by now, but if it is damaged it'll probably fail regardless. I think your odds are better in replacing the fluid and preventing further damage, rather than hoping the sludge will hold things together for long.

Of course, it's your truck; and your money!

Good luck,
MrBSS