17" spare for 18" tires

I wouldn't do that. Even in 2 WD, there's still a differential between the wheels on each axle, and the difference in rolling diameter will have that differential working like mad. It is going to get hot. And while you could probably get away with it for 5 or 10 miles at city-street speeds, take it much farther, or up on the freeway, and you'll be risking serious damage.

I know. I had a differential explode in flames on me once. So, been there, done that, ended up costing over $1500 to fix. And that was 15 years ago.