Sorry, just saw this. From Silverton to Animas Forks, via Co Rd 2, is just a rocky road. I think the biggest rocks you would see would be baseball/softball sized. I've not been to this area in a couple of years, so I just checked Google Maps and someone has driven nearly all the trails out there. So you can virtually drive all of these trails now. Have a look and see if any of it gives you pause.
As for the highway tires, traction should not be an issue unless the weather is bad (it can change very fast up there) and as long as you watch what your sidewalls rub on, those should be fine as well.