We've made that trip twice towing our 2009 Ranger 4x4 and I would not attempt it without towed brakes! You are likely to see some 10% grades with numerous 1 mile plus 6-8% percent. Yes, you can make it without towed brakes if you start down the hills VERY slowly with your engine brake fully engaged in 1st or 2nd gear. But what are you going to do when a moose runs out in front of you and you need to make a quick stop or a huge frost heave suddenly appears and you are traveling faster than you should?
Supplemental braking systems are expensive but you should keep I mind if you have an accident and the state or providence you are located requires brakes on your towed your insurance company may not pay your claim!
Take a look at Ready Brake. We have been well satisfied with ours and they are reasonably priced.
Hope you have as much fun as we have had on our trips up and down the Alaskan Highway!