The question was "Are there unique differences in the year that can be identified externally between 86-90 Comanche pioneers?" not "would anyone notice."
My '90 has a 10 slot grill, that does not mean it came from the factory that way. '86 & '87 had 10 slot grills.
Front grill is 10 slot for 87, if bench seat '87 did not have head rests, and as mentioned some paint and interior color differences.
Your truck has the Marchal lights which is an '87 thing, so that's good. Just swap your grill.
This is an Eliminator, the 6 in position six of the VIN designates that.
@jdog From '90 on the body code for Eliminators contained an "L", before that it was an "S".