You absolutely DO NOT need a permanent resident card to cross a land border by car. We have an employee who refused to get their card and flys out of Bellingham all the time. Commercial Carrier is Boat, Plane, etc. Not a car. They might give you grief that you should have it, but it's not required. Couple of months ago I came into Vancouver on a cruise ship and didn't have mine, I had all my documentation that I was a landed immigrant and worried for weeks, they just waived me through.