Growing up our neighbors had a similar issue. For him he installed a hitch on the front of his truck, he also laid out a couple of 2x10s but he carved out some 4x4's to provide support under the boards.
Today I think the first thing I'd consider is flipping the axles, if that's an option that would be a good choice. Then just need to address adding another step to your entry.
I'd echo Lynnmor's comment about the casters, those things are just asking for problems for your rig down the road (or sooner).