Sounds like modifying your Grove cable to connect the resistor pair to your differential input is your best bet. Here’s a schematic of the multi-purpose screw terminal board if that helps. The gray area on the 3 solder jumpers in the schematic shows the default position of the jumpers. If you’re putting resistors in R1 and R2, you’ll want to change JP1 to the res-div setting. Were you successful at moving the solder blob?