it is a 91 chev 4x4 so that makes it a k? but it says Silverado on the side of the truck so which is it? i go to order parts i am told there is no such thing as a 91 silverado...

 

Silverado is the trim package. Paint, interior, those sorts of things.

 

The C series for these trucks were two wheel drive and the K series was for four wheel drive. The C/K just represents the drive train of the vehicle.

 

they pretty much hit the nail on the head.
c/k were also used on the 73-86 old squares in 88 they came out with the style you have till 98 BUT from 87-91 all burbs, blazers, crew cabs and 1 tons were the old square style but they were called "R" and "V" for 2wd and 4wd respectively. your style even ran till 99 but that was the cross over years. dont quite know the year but in the early 2000s they just called them all "Silverado" regardless if it was a 4wd or 2wd or had all the bells and whistles, thats where the other package names came from like LS, LS1