I have a 1990 Isuzu Trooper with the 2.8 engine w/ 5-speed in it.  This engine is a Chevy 2.8 similar to the ones put in the S-10's and S-15's.  My engine has about 140k miles on it and is tired.  Chevrolet makes a 3.4 liter performance engine that is a replacement for the 2.8.  Does anyone out here have any information on whether this might be a bolt in replacement?  Would appreciate any info anyone might have.  

Theres quite a few people who have done it.  Im not one of them though.  If your looking for a good site with lots of helpful info, go to
http://www.4x4wire.com/forums/ubbthreads.php?Cat=&C=15   Then search the threads for any info your looking for.  The 3.4 is almost a bolt in swap.  You do have a little bit of modifying to do.  I believe the starter on the 3.4 is on the opposite side, so you have to swap it over to the other side.  Other than that, everything else should work.  While your at it, I would recommend getting either a Holley TBI unit or a 4.3 TBI unit and having the intake manifold bored out to match the throttle body diameter.  That would yield a good amount of power there.  But the guys on the Isuzu 4x4 wire are really knowledgeable and willing to help.  Just make sure you do a search first.