You may have a problem renting a car in Skagway. Skagway is a very, very small town, population 100 in winter. As soon as you drive out of town, you have to cross into Canadian customs. I really am not sure why would want to rent a car in that neck of the woods anyhow. There is almost no place you can drive. You go from Scagway to Whitehorse. Whitehorse to Dawson City etc. Greyhound runs up to Whitehorse and there is bus service from Scagwau to Whitehorse. I am not familar with Murray's highway log. Just about everybody who drives Alaska-Canada uses The Milepost, sold at Barnes and Noble, etc.
Definitely rent a car ASAP. Our group has 12 making the drive, and did not want to use a van (easier to get in and out of a four-door car). Therefore we reserved three cars from Avis (online) and also called to verify with the Skagway Avis office directly about a month ago. The photos shared by recent cruisers look terrific, and we are very excited about this excursion! We'll be making the drive next week while in port in Skagway.
Absolutely one of the best things to do while in Skagway - we reserved a car about 6 mos. in advance. Easy walking distance from dock to (think it was Avis or Hertz) rental agency.
Murray's Milepost marker guide is an ABSOLUTE MUST for this drive. It's invaluable - you'll know exactly what you are looking at when you stop at specific mile markers along the road to Emerald Lake. This is probably the ONLY place I'd actually want to rent a car during an Alaska cruise.