In pre-92 Troopers, you are better off buying a 4cyl if you can find
one.  The 2.6L 4cyl is an Isuzu built, overhead cam, fuel injected engine.
The 2.8L (or 3.1L) v-6 is a piece of garbage pushrod engine built by GM.
The real shame is that the 2.6L 4cyl and 2.8L 6cyl are both rated at the
same horsepower (120bhp) and almost identical torque figures but the 4cyl
reaches it's torque peak at a much lower RPM yet will rev higher than the
V-6.  I don't know the exact figures for the 3.1L but know that it was only
moderately more powerful.   I had an 89 4cyl manual that was dead reliable
and would out accelerate and out haul either GM V-6
   To answer your question, I would expect a 90 v-6 Trooper to be slower
than your Nissan Frontier.  If you do decide to go with it, you shouldn't
have any problem finding parts as it is the same engine used in numerous GM
products such as Cavaliers, S-10 Blazers and pickups,Cutlass, and even some
Jeep Cherokees.