Two wheel or four wheel anti-lock brakes

Well there's no such thing as Front ONLY ABS, So look at front brakes and see if there's a wire going there besides the brake hose, it there is, then it's 4W ABS otherwise it's 2W ABS or NONE. I don't recall ever seeing a 2W ABS system on a S-10! You'd normally see that on the Full Size trucks. The older full size trucks.

Well just so you know, my 91 S-10 is rear wheel antilock.

Whitelightning

just curious, is it a 4WD?

The S-10s P/Ups were available with rear wheel antilock in 2 and 4 wheel drive. The Blazers were the same BUT they also had three different ABS systems depending on year and options. Then there is the Bravada which was 4 wheel antilock.

Another item is that when in 2 wheel drive the 4 wheel systems work properly, when switched to 4 wheel it shuts off the 4WAL and makes it rear wheel only. This was a recall item on some because they changed it midyear due to problems with the 4WAL system causing problems.