Vibration 45-50

66 Dart, 833od. I have a 3rd/4th gear vibration while cruising, more vibration in 4th depending on how much I push the gas pedal. It's not a revolving type vibration, its just there if that makes any sense.

Has a new balanced 3inch steel driveshaft, has a shortened 8 3/4 with super quiet 3.55 sure grip gears,has diff additive, has dr diff axles. The Vibration happens when I hit 40-45 in 3rd or 4th, worse in 4th, but when I put in Neutral it seems to diminish or go away mostly but when giving throttle it comes back and when cruising it will stay there. Would the shortened 8 3/4 be causing this say if the pinion angle were wrong? I don't think the drive shaft is the issue since the car is smooth under 40 and vibration almost goes away when in N. Also have new Jegs wheels/tires/ balanced.