I had the same noise on my Fairlane that ended up being the driveshaft. I changed gear ratios from a 4.86 to a 5.14 and it moved the 59" long drive shaft into the critical speed. I had the original shaft balanced and checked twice at different shops, had another shaft built out lighter and larger material, Replaced several tail shaft bushing ( the vibrations was eating them up} removed the tranny and replace all the bushings, new yoke, had the rear end apart, looked at axle bearings...was getting very frustrated. Finally, there was a NASCAR team doing some testing at the Proving grounds were I worked and I ask them who built there drive shafts, They told me about a shop in Michingan, Dynotech Drive shafts, Called them, the guy I talked too guaranteed that he could build me drive shaft that wouldn't vibrate, and he did.
Mark Williams has a good tech article on drive shafts and critical speed
http://www.markwilliams.com/driveshafttech.aspx