Wondering if anyone has experienced this problem with ABS..

My 2000 Silverado xcab 4x4's ABS engages whenever I'm slowing to a stop (i.e. 10 -> 0). This happens at every stop, regardless of surface conditions. If I'm more aggressive on the stop, the ABS does not engage. I have 285 MTs on the truck but this problem started well after changing from OEM 265 tires. The only off-roading I've ever done was my muddy semi-gravel driveway a few times (got bad enough for MTs though)..

Damaged ABS sensor? Dirty sensor? Something more serious?

Anyone have an idea where I can find instructions on testing an ABS sensor (I assume this can be done w/ a multimeter?) I've yet to get to a bookstore for a Chiltons e.t.c.

Thanks in advance!
-Brian