I kust checked the owners manual on the 20076 Trailblazer I just got. It says change the spark plugs every 100,000 miles. It doesn't say anything in the maintenance schedule about checking them more often. Does mention that the gap is .040" -- that's in a 5.3L V8.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.