My understanding is that it is Federal rules, but the states can be more strict, just not less than the Feds. The limit is 26,000. That is the amount of weight that you are capable of hauling (GVWR), not what you have it registered for or running empty.
The only exemptions are farmers but there are distance restrictions and you must be registered to a farm. Most are 150 miles from your residence and/or in state driving only. If you go out of state, you are required to buy fuel stickers, and all of the stuff truckers have to do.
