I have a 1995 Legacy wagon and the brakes were worn so I took it into the shop for service. I also mentioned that the ABS light came on very intermittantly. They did the brake work then hooked it up to the diagnostic and it said the ADSL codes were #27,#28 L/Rear speed sensor.
They wanted $400 to replace it. I said no thanks.
The light started coming on more frequently, so I did further research-
Problem solved, cost $0. Though I did spend several hours searching the net for the info. I hope this saves someone else the trouble.