If you are lucky the rear was made ready for ABS and you just need to get a sensor or some axle rings.
If your car has 3 ch ABS you will need to pull the posi unit out, install the ABS reluctor ring and set the rear up again.
Also you need to drill the hole for the ABS sensor in the exact location to pick the signal from the ring.
If your car has 4 ch ABS you need axles with a machined area to press on the ABS exciter rings. Pull the axles, pull the backing plates,installl the rings, press the bearings and install.
For $100 let them do it - whoever told you that price either did not understand your need or are very cheap!
"Stop reading tech - you just learn things that you'll need to forget"