It's "Eyes" are the speed sensors on the Axles themselves.
It "Sees" one or more sensor(s) reporting a different deceleration rate from the
others and takes action.
SO,Either It's "Off it's Feed"
OR it's telling you ONE or MORE WHEELS are out of synchronous Braking action.


The front brakes groaning,along with the ABS kicking on, is the "Lotto Win" for
you diagnostically...Unfortunately, I am not expert DIY enough to be able to interpret the SIGNS.

I would repost this over on the TECH side with the new info.
stevebfl,AD or P.Hiedary may have run into this and know the exact cause.

__________________
'84 300SD sold
124.128