2 possibilities :
1. the wheel speed sensors ( get a cheap digital Oscope off ebay and have a look at the signals at the antilock btake module ( it communicates with the BCM via CAN bus )
2. The speed sensor in the transmission. This I don't know too much about, I know it feeds the TCM, and I would assume it would also get to the BCM via CAN bus.
https://www.ebay.com/i/283538347896?...saAt9qEALw_wcB