'99 Explorer - ABS and Traction Control bugs

I'm having two problems:

In Auto setting, my 4WD does not engage when my rear wheels spin. It gets surprising sometimes. 4WD High and Low both work fine when selected. Any ideas?

Also, while in or out of 4WD, if I do an excessive amount of spinning, the ABS light comes on and the ABS system disengages. It resets after the ignition is turned off and turned on and works fine afterward. Any ideas here too?

Thanks.
Curt.

May have a rear speed sensor concern. Check the reisitance on it (it's in your rear axle) my rule of thumb is 800-3500 ohms, any more or less and I just put the 18 dollar part on.