ABS Brakes

Question for anyone with the service knowledge to answer. I have a 2000 chevy truck with a 454 engine and 72K miles on it. The ABS light has been coming on intermittently. I took it to the dealer. Now I have a general distrust for deallers but the brake shops generaly do not have the equipment to diagnose an ABS brake issue. The dealer service manager said the front brakes required repalcement and rotors could not be machined any further requiring replacement (at 70K ?) and the front left wheel seal was leaking requiring replacement. In addition they stated the master cylinder needed to be replaced. I told them no on the master cylinder but grudgingly agreed to replacement of the rotors and seal. I made the appointment for a Tuesday and didn't, get the truck back untilt he following friday. Now I have the truck back and am backing out of my driveway first thing in the morning and the ABS light comes on. I drive it for about a mile and it goes out. I brought it back into the dealer service department and they said it is a speed sensor in the differential in  need of replacement. So now the questions; Why where they not able to determine this the first time around and is this an accurate dianosise. Can any one help?
Thank you