I haven't seen a Dart or Avenger, but haven't really been looking either. - Have you tried any of the die cast manufacturer's sites?
The only stores that carry diecast or plastic models around me are Hobby Lobby, Michael's, Toys R Us and..... well, that's about it! *lol*
Maisto makes a 2011+ Jeep Grand Cherokee in 1:24 Scale, its a part of their Special Edition Lineup (I've got one in Inferno Red Crystal Pearl). they also have 3 Generations of Vipers to choose from in the same 1:24 lineup: the 1997 R/T10, the 2003 SRT10, and the Brand Spanking New 2013 SRT Viper.