I'm not calling you a liar, I simply think you're misunderstanding the definition of moment of inertia because the word polar is being incorrectly placed in front. I should've caught it sooner, but my last statics/dynamics class was several years ago, so I'm a bit rusty. What we're really talking about here is a moment of inertia (no "polar"), and the idea, is not to remove weight from the poles.
The idea is to have equal weight at each pole, so that the item you're talking about is balanced. That's when the moment of inertia is low, because one pole isn't trying to swap ends with the other.
Since my last analogy with the scales wasn't effective, let me try again using the barbell analogy from the text you posted. The reason a barbell has a low moment of inertia, is because the weights on each end are equal. Removing weight from one end of the barbell would move the center of mass (aka centroid) closer to the heavier side, and increase the moment.
In terms of your truck, one side was already heavier to begin with, so it wasn't well balanced. The center of mass gets closer to the heavier side as the difference between the weights increases. So, you made the rear even lighter than it already was, which moved the center of mass even more forward than it already was. This INCREASES the moment of inertia, and as you already posted, increase moment of inertia leads to worse handling.
Now, this can be corrected by adding weight back to the rear, (which you've already done, and you've admitted that it helped) or by reducing the weight on the heavier end of the truck, the front.
I'm really not trying to offend you. Again, if you're happier with your truck now than when you started, good for you. Carry on.
