The Trooper was discontinued before the 3.5DI came out, and if you're going to make a VX-4 you might as well get the cool engine. I think there's a lot more commonality between the Axiom than the Trooper as far as the body goes. Plus, if I'm not mistaken, the Axiom suspension geometry is closer to the VX than the Trooper.

As an aside: How many families have VX's and no issues with kids? My wife drives the IronMan and our daughter hasn't ever been a issue as far as the truck goes. The real trick with a child seat is accessing it from the rear rather than wrestling from the front. Just fold down (or back) the unused half of the seat to get to the little one. Once they're older they scamper in-n-out like crazy and operate the passenger seat with ease. Of course, having a little bit more separation between 2 or 3 giggling little girls might not be a bad things...