They spin going up hill because they can't get any traction, doesn't matter about weight if you can't get the traction. Look at people who think their heavy vehicles can stop on ice and take off sliding. A little damp grass is like ice.

The dual conversions are out there for people that need the weight carrying ability for pulling a large 5th wheel trailer or carrying a truck camper or dump bed. My father converted a truck to dual wheels for use on the farm and we had to tow it out of everything so he pulled the bed off of that truck and sold it then put it on a 4x4 truck. You'll notice in winter the big 18 wheelers get stuck real easily or slide and jackknife even with all their weight because the weight is spread between so many tiny patches of wheel touching the ground.