50 Litre Tank in the boot, Thats going to add a lot of weight transferrance to the back end. Do you still have a standard tank as well. Consider this when you re-distribute the weight. It might not be best to put it exactly centre.
Point duly taken about not being able to move it yet.
From reading the whole thread again, theres a few ideas being banded about as to what it could be.
My personal stance, would be process of elimination. Change one thing at a time until you narrow it down starting with moving the tank, then trying a stock set of wheels, etc etc.
If your still unable to move the tank for whatever reason, pop something heavy in the otherside of the boot to compensate for the weight as a tempory measure.
My moneys on the tank and the tyres.
50 Litres of LPG swishing about will have enough momentum to knock a man off his feet.
Cheers
A