vibration

Hello all, Haven't been on in a while, but have a problem that's got me stumped. I have a 73 d200 pickup with a 318 and 727, it has developed a vibration at around 35-40 mph. especially when I let off the gas coasting down hill. I figure it has to do with the engine. I found one motor mount broke so I changed both of those, I got a new tranny mount that I need to get in still. I put a new carb and new wires on the engine and that helped a little. So far everything I've done hasn't changed much as far as the vibration goes, so I figured I'd see if anyone had any other ideas.