I did a write up on the How To for underslinging your axle, just not sure where it is so you may have to search..............the bit about causing swaying and such is really BS if it's done properly, you will gain about 100mm, sometimes a bit less so measuring things up before hand is needed...............for starters with your van level measure between ground and the bottom of your hitch then with your tug measure from ground to bottom of ball, take away about 25mm, this will show where your tug hitch would roughly be sitting when loaded up with van, add 100mm to the vans measurement it should be very close to the tug measure.
If its more than 25mm you may have to change the profile on your tyres to lower or raise, mind depending on load/springs you may only get 80mm from rolling the axle but it is doable and I reckon a worthwhile mod......I will see if I can find a link....there is a certain way it must be done and does require some measuring etc, you cannot just rotate the axle, the brakes have to stay in the same position, the spacers need to be repositioned with care to keep the alighnment, it is an easy job ......just need jack stands, new U bolts, fish plates and spacers, tape measure and patience.