ranchero64

For about 6 months, my truck has been shifting extremely HARD, and chirping the tires around town between 1/2, (no tune, normal driving) Ive also noticed a drop in MPG, 15 mpg down to 13, everything else "seemed" to be ok.
I tow a 4500 lb trailer about 60 miles once a month, and 1000 miles once a year. I got the truck at 59k, The truck only has 74k on it now, the fluid was flushed and changed at 65k.
I finally took it to a dealer, it was still under the dealer extended warranty. The dealer had it a week before they pulled the pan, they said the fluid "was filled with metal shavings" and that they were just going to replace the whole trans (not a rebuild)

Questions-
Well this doesn't seem normal. What could have caused this?
What can I do in the future to prevent this? Ill do anything. Coolers, external filters, change the fluid more often?
Why would they replace the whole thing? Fine by me, just seems like an expense repair for not investigating deeper.

Thanks
EP

2009 Silverado 75,000 Miles
MagnaFlow Exhaust
KN Filter/MIT Tube