Transmission vibration?

Hey guys, I just recently did swap from a 318 to 360 behind a 904 trans in my 76 dart. I've been driving it around for about a week no problems, but recently I've been getting this vibration at 40mph or when the tranny shifts into second. It seems to get worse as I accelerate and it's almost like I'm driving over little speed bumps. It also goes away if I shift into neutral. Any ideas what this could be?