Stick with the last, Swamper Radials if ya don't mind the coin.
Thornbirds are junk and you probably arn't going to want to run bias-ply on the road.
Just another sugestion, find yourself another set of rims for the Swampers and swap over to them when your are heading for the trail. If I recall you mentioned somewhere it was about 2 hrs to get to the cliffs. Thats quite a bit of pavement wear to be putting on them at $160 a crack.