Not free I'm afraid Bill, but you already own it ... FS9!
Not randomly I admit, but you can change the Payload and Fuel weights of your choice of aircraft under Aircraft\Fuel and Payload. Because the two are dependent on one another for flight planning purposes there is little time saving involved in randomising the pax and cargo weights.
Plenty of free flight planners around eg. vroute.
Apologies if you already realised this, but sometimes familiarity does breed contempt. :rolleyes: