Brake fluid needs to be checked. Sounds like its leaking from the line

Little info on front and rear info.
Front
https://www.youtube.com/watch?v=ZE_dDlzS4TA
https://www.youtube.com/watch?v=tmU836lJ7cY
https://www.youtube.com/watch?v=4wWmEWvInyM7
https://www.youtube.com/watch?v=gNinqsyn80g
https://www.youtube.com/watch?v=WvRyg95cUIQ
https://www.youtube.com/watch?v=ALcA-bQV4G4
https://www.youtube.com/watch?v=kPze_fn6c4g&t=12s
rear below
https://www.youtube.com/watch?v=p8k5GfTpZr4
https://www.youtube.com/watch?v=wZAMpsgdEO0
https://www.youtube.com/watch?v=Kc3qDLgcNMU
https://www.youtube.com/watch?v=fiVxRBOA-C8
https://www.youtube.com/watch?v=_-XnccEwpwc
https://www.youtube.com/watch?v=pHYCCyzE6DI
https://www.youtube.com/watch?v=yIT8djTxt6o
https://www.youtube.com/watch?v=_Mmp8UO7lNs
https://www.youtube.com/watch?v=8nRd8R_5lcg
https://www.youtube.com/watch?v=rY_Q2Z10C-Y

I assume your referring to the rear wheels the abs module is mounted on top of the axle in center between the wheels. You more likely to have a loose pinion . The nut most be checked ASAP. The drive shaft has to be removed to tighten. First find the plug on the rear end either a rubber plug you can see from the reason car looking at the rear axle. Sometimes a square plug or a bolt head . If the rear axle cover is smooth the the plug will be on the side of the rear end close to the driveshaft . Either way your leak needs immediate attention or your rearend can fail leaving you stranded and expensive to fix. It will need gear oil check your manual for the exact type. But if any of this is too involved take it to a reputable repair shop it will save you a lot of money because by now you are wearing the bearings causing the abs to light up

This is an accident waiting to happen check your fluids immediately.

Yes it could be check fluid immediately in fear of an accident.