Can't answer for SC but here's for IL.
It doesn't matter what the door sticker says when getting registration. The registration weight is a fee for the weight limit you intend to carry. If you never intend to carry a total weight (cargo and truck) over 8000 lbs then you register for 8000 lbs. If you think you might carry 12,000 lbs then you pay for 12,000 lb registration. If you never intend to carry 12,000 lbs then it's silly to pay the higher fee for a 12,000 registration when you could get by just paying for the 8000 lb fee.
You can register you truck for whatever weight you want if you want to pay the state the registration fee. There's nothing preventing a person from registering a pickup for 80,000 lbs if you don't mind paying the state all the extra money. You would never carry 80,000 lbs in a 2 axle pickup but if want to pay the state the extra money they'll be happy to take your money.
Registration is just the fee you pay for carrying the weight you intend to carry. What the door sticker says doesn't matter.
