dodge 3500 cummins clutch fan engages on flat with no load

I wouldn't say definitely that your radiation is blocked "on the inside" yet ... as noted somewhere up above, the RAMs (maybe other brands, too, but particularly RAM Cummins) are noted for coating the radiator with grime between the intercooler and radiator.

My Cummins performance shop friend ALWAYS uses a Hotsy pressure washer with degreaser to clean up the exterior of the radiator when chasing down this kind of problems (unless the internal corrosion buildup is visible like mine was.)