Brakes/sudden slip

This condition has happened now both forward and backward at slow speed around 10 to 15 mph on my 2000 w/61,000. I was applying my brakes moving slow in the parking lot when suddenly I heard a clunk and it felt like I hit a patch of ice the van kind of skidded forward/backward for 6 inches or a foot and then caught again and came to a stop. This has not happened on the road only when I first get in the van and drive in the parking lot. Any thoughts on the cause? Thanks.

I am experiencing this SAME EXACT problem with '93 Olds w/140K miles.
Somebody surely has heard of this and knows what in particular may be failing ABS?

Thanks in advance!

Generally speaking, ABS problems like this are caused by a weak or failing ABS wheel speed sensor. They usually cause problems below 10mph. The sensor isn't strong enough to provide a signal to the EBCM so it thinks that wheel has locked up even though it hasn't. You will want to either: put it on an ABS capable scan tool and see which sensor is dropping out, Use a multimeter and measure the A/C milivolts at each of the wheel speed sensors when spinning the wheel by hand or pull the ABS fuse so you still have regular brakes but no ABS activation.