1.) Tell them it is a Mercury Mountaineer. The policy only applies to Explorers, so that should be enough to fool them. They ain't too bright.....
2.) Pick it up with another vehicle, or have a friend pick it up, then switch it over to your Explorer. When you bring it back, either do the same in reverse, or....... Bring it back with the Explorer, then loudly proclaim to the idiots at U-haul that the somehow your truck, and their trailer didn't magically burst into flames when connected together. Inform them that you had a witch doctor lift the Explorer "curse", and they can now safely rent to Explorer owners. Don;t forget the obligatory sarcastic "Your Welcome" on the way out.
3.) You could try to reason with the manager, and explain that the "ban" was enacted over the roll-over issues related to the FIRST generation of Explorers, which haven't been made since 1994. Further explain that, other then the little plastic name badge glued onto the tailgate, your 2003 vehicle has virtually nothing in common with the offending model. It is a completely different design, front to back, from top to bottom.
4.) Further, if you wish to risk giving away the loophole, you could explain that they aren't prohibited from renting to Mountaineer owners, and they are exactly the same vehicle as your, with a different name glued on it. This tactic would give the manager the info he/she needs to conspire with you to "stick it" to the company, assuming they aren't very fond of their employer. Of course, it's u-haul, so it's a pretty safe bet that even the employees hate them. On the other hand you could end up with a stick-in-the-mud, company man, do-gooder.... In which case, I would just through a loud screaming, yelling, berating hissy fit, and at least make sure he goes home that night knowing what a tool he/she is.
