I have a Dodge 3500 2wd 5.9 12 valve dulley that starts to roars at 65mph but can be driven out of the roar at about 78mph

Check the gear box

Its called owning an old truck. with full floating rear axles. my 2005 truck roars at about those same speeds. Youre never going to fix it, because its actually the engine making the roar. I replaced my harmonic dampener with a "fluid Damper" http://www.xtremediesel.com/search.aspx? manufacturer=74&gclid=CNrJg_r-u7QCFQf0nAodKXcAlQ It took care of the noise some what. but with age these 5.9 engines just get louder and clankier. I also had to rebuild my rear axle because I use my truck to make about 1400 ftlbs of torque at the tires daily, and destroyed the stock rear end. after I rebuilt it the roaring went away for the most part for a little while then came back. You can also try rebalancing your tires. I doubt thats the problem though.

05 I had a one piece drive shaft for the rear no more noise