P1773 is indicating that a malfunction occurred in the ABS system that is outside the range of the enhanced OBD systems field of vision. - so sadly not a lot of help - however to go straight in a request a new ABS unit is the wrong way about it
its very likely a faulty ABS speed sensor. They are well known for failure on this model, particularly the right rear and the left front for no known reason. On extremely rare occasions the ABS pump unit can fail, but we are talking about 1/1000 cases.
At this point you are going to need to get the car to a shop that has a scan tool with full ABS/ASC support so that the fault codes can be identified and part replaced.
If you can not get to anyone that has a full scan tool (not just a code reader)
Then its back to checking wheel speed sensors as I've already mentioned
If you unplug each wheel speed sensor, you can probe the two wires of each connector (on the sensor end) with a meter set to AC voltage. In this state, if you spin the wheel at hand you will see voltage appear on the meter. Not much, but a little. In this respect you could check each sensor this way and if one shows 0 at all times with wheel spinning, then you know that sensor is bad.
Of course the ideal scenario would be to scan it with a proper scan tool, as there is still the slim chance something else apart from a speed sensor is amiss. As I said though, that is really the only common thing.
Maybe every once in a while we'll see the stop light switch malfunction (you wouldn't have any brake light control if this were the case)
The sensors are note terribly cheap on this model, averaging about $250-275 each. With four of them possible to go bad, plus the slight chance that there is something beyond that going on, it would be money well spent for a quick ABS/ASC scan to see what is going on.
Once the problem is corrected, the ABS capable scan tool will be required to clear the faults, and then the OBD faults (P1773) will need to be cleared as well.