I am getting an occasional ABS warning light at random times. All it takes to clear it is turn ignition off and restart and that takes care of it till maybe 2 weeks later. I am going to replace the rear speed sensor in the diff. and was wondering if I should unhook battery cable. Not sure if this sensor is powered with engine off (my thinking is it is not powered but just asking).Truck is 2002 F-150, 5.4l. if that matters.

 

Abs issues can be hard to diag even when looking at the car since their are a few items that can go wrong speed sensors are a good place to start, if that doesn't take care of it I would look at the mod itself.
A good rule is when ever doing anything that is moniterd by the pcm you should unhook the batt just to make sure it gets reset.