Diesel out of the radiator overflow????????? Not to doubt you or anything, but are you 100& sure its the radiator overflow you're looking at? That would be the large plastic container at the front center of the engine. In order to get diesel in the coolant bottle you'd most likely have a blow head gasket and/or injector problems. I've never heard of anyone ending up with diesel in the coolant. Engine oil can get in there if there are major problems though.
Now are you possibly looking at the passenger side fender well right behind the exhaust manifold at a little metal tube that just seems to stop suddenly? If so then you're looking at the fuel bowl drain line. What happens is the viton o-rings fail causing fuel to leak past them and then going out the drain line. Orings that had been run for long periods of time on LSD and then switched to ULSD have a tendency to fail. These orings can be had on ebay for around $5, and once changed out you shouldn't have a problem again now that LSD is getting harder to find.
99 F250 PSD 4x4, 5" DIY exhaust, CCV mod, DIY intake, PMT1 with DP Tuner chip (stock/40/80/120), BTS VB, Suncoast Extreme Trans with SD43 converter, FASS fuel system.
Trail Rig: 95 Ranger on Kingpin D60/14 bolt/D&D Doubler.