Legality has NOTHING to do with GVWR limits that are arbitrarily applied by the manufacturer. LEGALLY, as long as you are below your tire limits and paid registration weights, you are fine. If I register my Ford Ranger to 14,000 lbs and put E rated tires with 3500 lbs per tire limits, then I can LEGALLY haul 14,000 lbs of truck and payload. I doubt I would be able to move it very far before a lot of things started to break... but it would be 100% LEGAL.
The 3/4 ton designation of 10,000 lbs is simply a regulatory number. Some jurisdictions limit the ability to drive a vehicle over 10,000 lbs in certain lanes, on certain roads or over certain bridges. Some jurisdictions define any vehicle over 10,000 lbs as commercial vehicles and charge more registration fees. That doesn't make it illegal to carry more than 10,000 lbs though.