Best guess on my issue ?

I've got a '72 Dart that had the exact same issue. I'd get up to about 50 miles per hour and would get this droning vibration. I had everything checked or replaced. Drive shaft, rear end, exhaust. I thought it might be the tranny but that checked out. Finally I had the torque converter checked and sure enough, the previous owner had the wrong converter in it. It was out of a 360 which is balanced differently I guess, internal vs. external which ended up causing the droning. I thought balanced is balanced but apparently it isn't. The issue is gone. It runs smooth as silk down the road now. Check and make sure you have the right converter on your tranny.
I've got a '72 Dart that had the exact same problem. I tried everything, and I mean everything, rear end, exhaust, driveshaft, all of it. It ended up being the torque converter. I have a 340 and someone put a converter from a 360 on it which is balanced differently. New converter and the problem is gone.

rebeldart said

I've got a pulsing vibration most noticeable around 3500 rpm cruising. Doesn't really sound/feel like the rear end (all is new/rebuilt with 600 miles) I'm guessing maybe driveshaft which I had cut and balanced ain't right ? Definitely not the "howling" rearend noise - it's a droning (pulsing) vibration that I can't really pin point, kinda feel it throughout the trans tunnel/floorboard. Should I start with pulling the shaft and having them double check or is there somewhere else I may have an issue ?

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