Catastophic Drivetrain Failure

[zonefive]

I'm the original owner of a 2001 3.0 E53, auto trans, and for the first time ever, my car left me stranded. I was going over the Coronado Bay Bridge of all places and out of nowhere I heard a really loud screeching sound, worse with power application, and the car went from full power to absolute zero.

Park didn't even hold the car on the slope -- I had to engage the parking brake. The only thing I noticed before this happened was twice the morning of the failure, when I put the car in reverse, I felt a clunking sound once the car started moving, as though I had run over something. I even got out of the car to see what it was -- and there was nothing there.

I have absolutely no warning lights at all, and no leaks underneath the car which makes me wonder if it is a transmission problem -- usually when the tranny goes, there are warning lights and you can get home in limp mode, right?

Having my car towed to the mechanic on Monday. He says it is likely either the tranny or the transfer case. If it is the tranny, he recommends finding a good used one, but I'd like opinions on that -- the youngest of these cars is about 8 years old and I'd rather fix it right and not have to face this issue again in 30-50k miles (if the typical trans is good for 100-150k).

FWIW I have been changing the tranny fluid and the differential fluids every 30-40k, hoping this would extend the life indefinitely.....

Thanks for any thoughts on this matter.