Does a 1991 Chevy Silverado have posi?

Not all 1991 Chevy Silverados came with Posi-traction (a limited-slip differential). Whether or not a specific truck has it depends on the options it was equipped with when it was manufactured. You'd need to check the vehicle's documentation (owner's manual, window sticker if available) or look for the designation on the differential itself (usually stamped on the housing).