A few tire manufacturers make a 255/85-16. For your sake though, I agree with the above posts about the 235/85-16. Since you've got 3.55's, you could even step down to a 215/85-16 to improve your mileage a bit. I know with 235/85's and 4.10's, I'm running 2k RPMs at 60 MPH. Your speed at 2k RPMs would drop to probably 67 or 68. If that's something you can live with, it may be a viable option (they're also a bit cheaper, too). The only downfall is that they will look small on the truck.