F450,
The reason most FWD vehicles are NOT towable 4-down is the way the transmission is operated and lubricated. SOME front-wheel drive vehicles ARE towable 4-down - you can review the various makes and models year by year with a simple search for the year in question. (My 2009 Saturn Outlook AWD is OK to be towed 4-down - but many other brands and models of various years are not - unless you invest in the addition of transmission fluid pumps or disconnect of the drive-train. (A major pain for either situation!)
So if your owner manual says not to do it - DON'T!! It could only take a few miles at high speed and you be looking at BIG $$$ to replace the tranny & perhaps other expensive stuff as well.