Have come late to this thread, but I wouldn't try to tell anybody as experienced as MTNGAL what to buy. I will however give a perspective on Alaska from 45 years experience. On the Alaska Highway you usually won't see any critters except near the road - they blend into the brush too well. You are likely to see critters & birds near the road; sometimes, like bison in BC, on the road. You may be able to see some sheep or goats up on a mountain side, especially at the visitor center at the east end of Kluane Lake in Yukon. So you will seldom need any more lens reach than to keep from being eaten or stomped. Most of that trip is wide angle country. The majority of the big animals are out very early or at dusk. There are a couple of overlooks west of Kluane where you can get jays to feed from your hand. Whitehorse is a good place to take a break from driving and check out the town (e.g., the old paddle wheel river boat) or restock the larder. If you have the time and inclination, a diversion to Skagway either coming or going (campground reservations are recommended) for the Klondike gold rush history and spectacular geology along the road.
I can't comment on the more recent situation along the highway from Tok to Fairbanks as we lived in Anchorage area and turned south at Tok. In any case, you better get the current edition of the "Milepost" guidebook. The drive from Tok to Anchorage is very scenic but there are stretches of white knuckle driving (for trailer folk) from Sheep Mountain to Palmer. The road is good; the drivers aren't. (In poor road situations, get behind a parade of big RV's, staying back to allow cars to pass easily.)
In our 45 years in Alaska, we traveled a lot but still didn't see it all - ain't possible. I've used a lot of cameras while there from a 4x5 view camera to film & digital pocket jobs, but always with a Pentax nearby. Most of my pix seem to be wide angle to medium telephoto.
I don't know what you are planning on for your trip, but if you want I can maybe provide some info. PM me.