1999 Jimmy 4wd... Recently, the yellow ABS light has been coming on upon startup...

When I start the vehicle, the ABS light is usually off. But after driving for a little bit (<1/4 mi), the light will flash, and then stay on. It will often stay on for the whole trip, but may clear. Many times, when it is on, I will be stopped, the red "Brake" light will flash, and then that and the ABS light will clear.

I've also had it when I will be applying brakes (dry pavement) and the ABS will kick in for no reason.

I've recently (within the past year) replaced both front hubs and speed sensors, so I know it probably isn't that.

The service (regular hydraulic) brakes work fine.

Any ideas? Anybody out there had this issue before? I'm thinking the ABS controller myself... If so, how hard (and $$$) is it to fix?

Thanks!