Vibration at Deceleration

I put a 340, 833, and 8 3/4" from a 72 Duster into my 68 Dart. I took it for it's first ride to get the front end aligned, and noticed a vibration. I got on the highway, went through the gears, after shifting to 4th and letting off the power it started vibrating. If I get back on the power it stops. It only vibrates at higher speeds in 3rd or 4th when I let off the power. It's at it's worst at about 60 mph in 4th gear coasting. The rear end is 3.23. This is my first restoration. I just swapped all the parts thinking it would work. What should I do now? Thanks.