I recently did the brakes on my P/U. (98' 1/2 Ton 4X4) Within a week the abs light and brake light came on. I haven't noticed any braking changes between front and rear, or left and right. No surging, or pulling either. I unhooked the battery to clear it, but it came back. I checked for leaks and checked the combo valve and the RWAL valve as well. Other than replacing valves I'm at wits end.....any ideas?

check for resistance at the rear wheel speed sensor on the diff housing

I'd have to say check the rear axle speed sensor also.
I had the same thing happen to my 99. the "abs" and "break" lights came on, then a little later the spedo quit working below 30 mph. the magnet on the rear axle speed sensor had picked up small metal shavings from the gear lube, and it wasn't getting a reading from the tone ring. you might be able to get by with just cleaning the sensor, but if you have to replace it, it's only like $12. it's also a good idea to change the gear lube to get all the shavings out of the dif, and check the gears for excessive wear.