do dogs grow out of rolling in smelly things. My dog is 18 months and for the past month or so he has been rolling constantly. Most of the time it is just grass, but if he can find anything smelly he will roll in that. I have had to bath him two days in a row this week because whatever he had rolled in was unbearable.
Definitely sounds (or smells) like fox poo, which is quite sour and totally rank when you are in a car with the offending "rollee". It's just something you have to live with - if it's any consolation, you will become a little more immune to the pong :)
Sorry Zach if it's anything like mine gets worse as they get older!! Lydia my eldest dog loves to roll in whatever she can, it's even better if she's just had a bath and she's 12 this year :d