A couple of issues to keep in mind. Not just the physical issues of transporting a 55 gallon drum of diesel, but the fire hazard created should that drum rupture in an accident and then - somehow - catch fire. Of course, it's not a combustion hazard like plain old gas, but the fire hazard is definitely something you should be prepared to deal with. The standard small extinguisher carried by some motorists won't do the trick with that much fuel. Then again, you probably can't carry enough fire retardant for 55 gallons of fuel. Any firemen among us to help us out here?
Also, should a spill occur with 50+ gallons of fuel, in Missouri you are required to notify the Department of Natural Resources ASAP upon discovery. There could be some costs for you associated with cleanup.
Fuel transport is a big responsibility and large quantities of fuel present a bigger opportunity for us to mess it up. The more volume of product, the easier it is to mess it up.
Now that I've thought about it, I'm probably not careful enough when carrying 10 gallons of diesel around in my truck. I might have to change my methods a bit.