Disconnect the electrical connector at each WSS and check the resistance across the terminals for the wires going to each WSS.
Do a continuity check for each wire from each WSS going back to the EBCM.
Keep in mind the SSS message can be initiated by more than just the ABS system and the WSS'*. I'm not sure on the Uplander, but on my Bonneville a bad steering wheel position sensor and a bad yaw sensor could both trigger the SSS light. The most common failures are a bad WSS, and lack of continuity to the WSS from the EBCM, which is why I suggest the first two steps.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)