differential problums 1994 GMC

hi i bought a 1994 GMC searra 1500 2 wheel drive a few months ago from a towing co. i noticed when i make hard turns the rear tires make screaching noises like im going 70 around the corner but im only going 15 mph it felt like full posi so i jacked up the truck and spun the tire and both turned the same direction so i looked for a switch on the dash and could not find one i have 33x11.5 tires on the truck and i dont think this could be doing my rearend any good i tride to find out what rearend it has all i could find on it was no. 5822 and xpkk1462 and gmd and theres nothing in my glove box anyone have any ideas. thanks

Glove box code for the locking diff is G80.