We have a 95 Ford F350 crew cab long box dually with a 7.3 litre turbo diesel and purchased a 25 ft. C-Dory in Port Alberni and brought it back to Alberta in June 09.
Coming up the Coquihalla, I was in the slow lane with the semi's, with the four way flashers on - passing them though! 8% grades slowed us to about 60 km/hr; 6% grades were about 80-90 km/hour. I don't want to think about the 10% grades.... We're running dual axles and electric brakes on the trailer, so are legal. I need to do more tuning of the brakes and myself, though.
British Columbia is pretty careful with its requirements, and we're legal with our combination. Smaller (eg. 3/4 tons et al) would need careful checking.
We're strictly two wheel drive and the truck doesn't get used much in winter (we live in Calgary) due to the complete lack of traction. We've just launched/pulled out in Port Alberni and at Ghost Reservoir in Alberta, but I want to rig out some alternatives (probably winch based) in case of traction problems at ramps.
I don't know how the dually compares for stability when towing, but - by reputation - they add significant lateral stability. Driving her without a trailer I sometimes find myself passing cars on turns as there is effectively no lean, but if those hard rubber rear tires ever let go, you're riding a 7000 pound barbell....
We're planning on bringing the C-Dory back out to the coast and exploring the Broken Island Group next summer, with the plan to dry land store her and rent tow vehicles as required. It appears that you can't rent a tow vehicle in Alberta, but can in British Columbia, so that is an alternative to spending the Big Bucks to have a BC based tow vehicle. I don't think we can afford the fuel to drive our truck out to the coast and back on a regular basis!
Later,
Doug & Fran