Well I'm glad you're OK. Without having been there, a couple ideas come to mind.

How much weight did you have in the bed? Trucks need some weight back there to get the most out of the rear wheels, usually a couple of sand bags do the trick (plus you can open one up if you get stuck).

What kind of tires are you running? Summers are a bad choice, you can get away with all seasons with some experience, but winters/studs are the way to go.

Sometimes a fishtail happens when you hit the gas a bit too hard on a curve. Again, I wasn't there, but I've done it.

And then sometimes you do everything right and there's just a spot on the road that you can't do much about except react.

Hopefully something in here helps.