Moving tandem axles
Here's where the moving of the axles comes in. If you are legal overall but your rear axles are overweight, you need to redistribute it toward the front. How can that be done? Well, remember when you were young and played on a see-saw which could be moved in the middle? If you and your friend stayed there on either end but moved the middle, you would have to work harder on one end to push and vice versa on the opposite end. So it is with the axles. If you are over on the rear, you need to reduce the rear weight by moving the axles back, placing more of the total trailer weight on the front of it. You do that by setting the trailer brakes and pulling the pin which locks the axles in place. You then move the truck and trailer forward while the axles sit still. The holes are generally placed so that each one changes the weight by about 400#, so if you're 1,000 over on the rear, you would move forward about 3 holes. If you're over on the drive (dual truck) axles, you do the opposite by backing up.
After you move them and relock the pin, you need to reweigh to see if you are indeed legal now. At many truck stops, the scales give you axle weights with separate platforms, but on a single platform scale, you need to get a total weight, then progressively pull off the scale with one axle or set of axles at a time to get weights and then do the math to see if you have done what you need to do.
Many states allow up to 1,000 over on any axle or set but some don't, so you're best to get it right. If you can't move it enough, then you have to get reloaded with the freight moved around inside the trailer and start the weigh/move process all over again.
On older trailers, you sometimes can't unlock the pin and move them, so many truckers try to avoid scales. The problem with that is that if you are caught doing it, your fine will be higher!
Another thing to remember is that some states regulate the length of the wheelbase of the trailer--in Michigan where I'm based, the distance from the kingpin (on the trailer where it mates with the fifth wheel on the truck, usually 3 feet from the front of the trailer) to the center of the axles at the rear can be no more than 40' 6". There's a lot to "juggle" to get your weight and length right.