93 zj ABS light on rear brakes lock up

Recently had my front drive shafts replaced at local 4x4 shop and not to far in the future my ABS light came on. Rear brakes lock up if barely touching pedal.

Mechanic said he hooked up scan tool and bla bla bla and gave a short shrug you may need to change Abs pump? Where should I start. Mostly stock except 3 1/2 lift.

Thanks