What diff. does my truck have??

[Joeyk94]

Yesterday I was out messing around and one of my friends joined in with his truck. As we left his house I was in front and decided to do a burnout down his street (young and dumb) probably about three houses down the truck hit second gear it started getting sideways. I'm pretty sure i have an open diff. it's a 97 Silverado 5.7 4l60e 3.73 5 lug on 35s. When we came back I put the truck in second and it spun both right away from a dead stop. This truck has only spun one wheel before when my dad used to have it and mess around. What differential does this truck have to make one wheel spin until it hits second gear. The rpo code says gt4

 

[83GMCK2500]

GT4 corresponds to the ratio, 3.73 in this case. Is it followed by G80? G80 corresponds to the factory locking differential. If you do not have the G80 code then the truck was built with an open differential, no telling if someone has swapped it out.

 

[Joeyk94]

Oh OK then all that comes after gt4 is jb5 and it's the stock different my dad bought this truck new in 97 and has never opened the pumpkin