Best guess on my issue ?

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 ?