Abs light on?

when my truck is on but in park the abs light is off but when i drive it turns on??(99 blazer)

It'll need to be scanned with a scanner that'll scan ABS to determine what the problem is. In the meanwhile you can make sure your master cylinder fluid reservoir level is high enough to cover the level sensor if so equipted.
You can also look at the wires on the front brake calipers for broken insulation.