Thanks for using JA
The anti lock module will record fault codes when it senses a malfunction in the system, therefor the module should be scanned for fault codes then fault traced
that is the proper process.
this could be a faulty speed sensor or a the module itself.
the shop also should drive the vehicle with the scanner connected to the vehicle to monitor all the wheels speed to see if there is a variation. other wise they'll never figure this out.