In my 30+ years, one of the biggest mistakes I see drivers, new and old make is when sliding axles on a trailer. The most common, and inertly dangerous way is the way I see most drivers trying to slide axles. They release the trailer axle locking pins, then climb back in the truck, set the trailer parking brakes and try pulling or pushing the trailer along the slider assembly. This is a very good way to damage the drive line in the truck, and you run the very real risk of doing some serious damage to the trailer as well. Other than trailers equipped with air lock pins, the best and easiest way to slide the axles, is to release the locking pins, then roll the truck and trailer back or forward as needed, then using the trailer hand valve (spike) to apply just the trailer brakes. The trailer axles will stop, and the momentum will move the trailer along the slider assembly. Then you set the parking brake, and go have look to see how far the axles have moved. The rule of thumb is, for every hole the slider assembly moves, 500 lbs moves with it. If you move the axles back 2 holes, then you have moved 1,000 lbs forward. Continue moving the axles back or forward as needed using this technique until you get the axles to within 1 hole of where you want them. Then you can release the pins, and very gently slide the trailer back or forward using the spike to lock the pins into the holes. Be sure to visually verify all the locking pins are engaged before driving off, and double check your axle weights as well.