Glad to hear that the stock tires do OK in the snow. Probably more a credit to the Tribeca's AWD system than it is to the tires- but if it works, great.
There is not a huge selection of tires in our 255/55-18 size, but one good web forum about it is our friends who own Touaregs. The v8 T-regs wear the same size shoes, and they are a bunch of car enthusiasts with an active forum.
So- check it out at:
http://forums.clubtouareg.com/forum2...6aa2241f19a3e3