ABS light on and off with a "clunk"

I hope one of you auto geniuses can help put the pieces of this puzzle together!:

Here's the deal.... My husband was driving to work yesterday, and the ABS light came on. He got to work, worked a few hours, came out and noticed he had a flat. He changed the tire. Since then, the ABS light has been going on and off as we drive (note: it came on before he noticed he had a flat). On occasion (this morning, it was around the time I put it in drive and accelerated after backing out of the driveway) we hear a "clunk" out of the van. All of these things are new -- just started yesterday. Here's my questions:

1. Auto Zone said they don't read ABS codes when my husband called. Where can we go to get a free reading?
2. Is there anyway possible that changing the tire caused the ABS light to come on? Are the two linked or are they simply separate incidences?
3. Why would the van be "clunking" all of a sudden in relation to these two items?

Has the tire been replaced with a matching tire or is the mini spare still on the van. If the mini is still on it may be because the diameter of the tire is smaller and the ABS and if you have it Traction control senses the wheel speeds differ.

That is excellent thinking. We actually had a full-size spare on it until yesterday when Discount Tire patched up the regular tire (which does match the other tires). If the psi of the tires does not match for all four of them, would this be messing with the ABS traction control senses? Could the ABS light have come on before my husband noticed the flat because the tire was STARTING to get flatter than the other tires and thus made the tires unequal in psi? Are they that sensitive? By the way, the ABS light was off this morning when we drove it five minutes down the road and back so maybe it's happy again since we had the old tire put back on. This is our only vehicle for our family so it is important to us to make sure everything is okay....