I have never had any problems with rear wheel traction on my 4x4 single cab but it is running on standard specification tyres with the recommended pressures in the handbook.
A empty ute with a tub or genuine manufacturer approved aluminium tray will most likely have more weight on the front wheels than the rear. The recommended pressures will ensure the slip angle of the front tyres will be slightly higher than the rear tyres. The slip angle is the angle between where the wheel is pointing and where the distorted tyre is pointing. Having the front angle slightly higher than the rear will ensure the front of the car will understeer i.e run out a little wider in corners rather than have the rear end go first which is oversteer. This is done because very few drivers can handle the rear end sliding out first.
As you keep loading the car, the load on the rear tyres will increase. As you increase the load on a tyre in a corner, you reduce its traction to the point where it will let go and slide. This means the rear of the ute should now slide out first but it doesn't because you have to increase the rear tyre pressures in accordance with the load. This reduces their slip angle to the point where, when everything is loaded up correctly, the front slip angle should still be slightly higher than the rear and the car will continue to understeer.
The moment you change tyre sizes and pressures, all of this design work carried out by the manufacturer at a cost of millions over years of development goes out the window. Who knows what the car will do now.
You may also find the front pressure does not change at any time regardless of load. That applies to my car and possibly many others. If this is the case then it explains why many owners have fitted a steel bull bar and winch and found the front axle weight is now over its maximum capacity. The front end is not supposed to have any weigh added to it by anything other than a manufacturer approved accessory. By weighing it down the angles of the front wheels change immediately and they will change again, to the wrong position, in corners.