Thanks all! I did verify that I have the Mazda M4-121G engine yesterday. It took a lot of digging on line. I read a TON of stuff and finally ran across some similar posts and other technical specs that led me to that answer. Getting verification here was the icing on the cake. Thanks again.

It's still crazy to me how difficult it is to figure out basic details, even with serial numbers, etc. I guess it is more about protecting the service industry than transparency. Imagine that!

GM was commonly used in that year model truck but the mazda was also an option.
There were 3 GM engine models used: 4-153, 4-181 AND 3.0L engines and only 1 Mazda model engine M4-121G

one way to tell is to look at the valve cover

The GM valve cover is long straight and has filler cap near the rear of the cover and PCV valve at the other end of the cover.

The Mazda (M4-121G) engine is different in that it has a slant to the valve cover, a breather in the middle in the slanted part and the filler cap on the rear side of the valve cover.

Note: orientation may sound confusing but when i say "rear" of the engine it is the "actual" rear of the engine. It does sit in the truck backwards so keep this in mind.

I looked that model up on X Part and the motor options are GM 3.0, Isuzu diesel, or Mazda. The older model S50XL used a GM 2.5, but not the one you listed.

Are you sure? Why do you say that?

I will clean the block, etc tomorrow, so I should be able to get some definitive answer, but just want to know if there is some easy way to know for sure.

That's sounds right.