So i have a 1990 k1500 4x4 with(had) a 10 bolt rear.

I'm putting in a 14 bolt/6 lug from a 97..

My ? is when i took the diff cover off there is some sort of locker/posi in there.

Did gm have a optional locker for this year?

Is it worth keeping?I plan on bolting it in tomorrow.

Thanx..  

The Posi in it is a a GM GovLoc. While some like to call it a locker, it is a clutch based LSD. While GovLoc tends to "grenade" in 10 bolt in hard usage with bigger tires, the one in the 14 bolt 9.5 semi floater is a lot more sturdy. The extra size and diameter on carrier in 9.5 makes a lot of difference. BTW, very smart up grade. The 9.5 is often overlooked for 10.5 but it is a very viable upgrade for 1/2 tons.  

Thanx sno.Is there a certain fluid i should run in the unit?  

Off the shelve 80w90 will work fine.