1992 Ford F250 ABS light and speedometer problem

When you turn on ignition rear ABS light comes on as it should. After starting engine, ABS goes out but comes back on 30 seconds later and stays on. I checked sensor on rear end and plug-ins and they seem ok. Also, the speedometer fluctuated when the speed was constant, which affected the cruise control. I have bled the brakes, shoes and pads about 60%, checked vacuum booster, and checked the vacuum pump (this is a 7.3 diesel). Also, the brakes are frine and feel as if the ABS is working.