Update. Although removal of the spacers helped, I was still not happy with the tracking after driving the Montero in a bunch of different conditions.
I got to looking more closely at the Hunter report, in particular the number for the toe-out on both rear wheels. After putting some pieces together, and considering the unsettling feeling that the rear end was "floating" around, it seemed that this was most likely the problem. That the rear tires were squealing in parking lots was also an indicator.
My main concern at this point was that the stock adjustments were maxed out, and this is what I was stuck with, as the alignment shop I took it to –on two separate occasions– said they'd "done all they could."
So I finally decided to take it to a different alignment shop to get a second opinion on how it was set up. They couldn't believe the toe on both wheels was that far out in the back. Although I neglected to look myself, they noted that the toe adjusters for both rear wheels were almost seized, and had probably never been touched recently, if ever. In other words, the previous shop out of laziness or whatever left them as is.
Long story short, the second shop got the whole thing back into the factory alignment specs, and the Montero is now planted solid in all conditions so far. It took some doing but the final outcome is superb. Absolutely rock solid with the new suspension.
Thanks again for the feedback and reading the thread everyone.