They are supposed to cross-reference if another vehicle's parts fit your application. I'd go to a local wrecking yard, where there should be no shortage of S-10 pickups and Blazers. Take a look at the manifold of a 2.8, they may be identical. I checked the Car-Part.com listings and they show a 2.8 Trooper to have the same, I expect they're correct. They also listed the specific manifold off a 3.1 Rodeo so why not just order that one?

You could also ask a local wrecking yard to check the interchange manual for you. There are lots of 2.8 parts kicking around so you should be able to get one very reasonably priced.