1998 Sierra 5.7 o2 sensor location

Hi All,
Can someone advise how many o2 sensors there are on my truck.
Today I guned it getting on the freeway, noticed smoke blowing out
the back. After parking a 1/2 hour, I noticed the Service Engine Light
come on after restart. Reading through this forum, I'm thinking that is what has caused
the light. Maybe not ? But the timing is strange. Have had no problems with the
truck.
If I read other posts correctly, there are 4 ? One on each side of the exhaust
before the Cat and one each side after ?
I have dual exhaust, bought it that way but don't know what kind.
Mileage is 92K.

Thanks

Why worry about the o2 sensors now, get the codes and post back.

Good advice Max,

Codes read EGR. I replaced along with the plugs.
Filled up with 89 Chevron but still pings a little on heavy
acceleration.

Any idea's ?

Thanks