Just reading the Diagnostics aids section, if your system has ever been apart, it's possible that someone did not put the pistons to the top of the modulator when putting it back together.

Normally, they sit at the top of the modulator and when they initialize, the move to the bottom, then back to the top. If two are at the bottom one at the top, then those codes could be thrown.

Just be careful if you pull the motor pack from the modulator without a Tech 1 or 2 to relieve tension on the gears. The motors have brakes on them and hold tension to the gears, when you pull the motor pack from the modulator without relieving tension, the gears spin at a rapid rate that could really injury fingers or any other piece of flesh that gets caught in them.

When putting the motor pack back on the modulator, make sure all the pistons are at the top of the bore.

Watch this...be sure to watch all of it. It will help you to better understand what is happening with the modulator. Nice little video. From the same group that did the Buick Know How Opti troubleshooting video. Sure it's focus is on Buicks, but the same system is used on 4th gen F-bodies up to '97. I think the ABS system changed in '98.