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I've read a few things on the web that drew my attention to a lesser known feature of the A4988 drivers. Delta printers tend to show artifacts or ripples on prints. The A4988 drivers have a mode that can be used by grounding Rosc (IC pin 13).

I decided to lift a heatsink on the unused Z2 stepper driver, and find out if it's easy to ground Rosc. It turns out, it's not super easy but if you can solder (or even just remove) SMD resistors, then bridge the gap with solder, that's what it takes.
I highly recommend trying the suggestion from user rq3: Leave the resistors on the board, just add solder on top to bridge them.

I began by taking photos then removing the Trigroilla Pro motherboard to work on it. Then I located and traced the Rosc pin to "R49_"... X, Y, Z and E. This resistor seems to connect between Rosc and Gnd, so bridging it is what I did.

I didn't get it right on the first try. One motor worked. I had the Z motor endstop in the probe endstop socket. After that, one motor still wouldn't go. The logical reason for that is if the solder bridge didn't make proper contact. It turns out, if Rosc is left floating that motor driver won't rotate. It will simply hold the motor in place.

I haven't confirmed if this did actually set the Rosc mode properly, but the printer still works afterward so I'll leave it like this.

A video of soldering is here: https://www.youtube.com/watch?v=blBsU985R-c&feature=youtu.be

The resistors are below the heatsinks, which stay in place.

Photos
Overview of the stepper chip: Thu_Jan_09_15-43-10.jpg
Chip closeups: Thu_Jan_09_15-40-42.jpg and Thu_Jan_09_15-40-35.jpg

Tracing the PCB track and via: Thu_Jan_09_15-42-11 is the top side
PCB from underneath, tracing the vias: Thu_Jan_09_15-41-41
Those last two photos are meant to indicate pin 13 goes to a via, then on the other side of the PCB it heads down to another via before emerging at R49.

There's a Gnd via to the right of the Rosc one. Both are right above the resistor (Thu_Jan_09_15-40-26.jpg and Thu_Jan_09_15-39-56.jpg)