When i was running my 700r4 at a certian speed it would vibrate, but if i put it into overdrive it would preaty much go away. Now i have a th350, new cross member, new tq arm, new shocks and springs i still have the same vibration, but if i put it into nuetrul it goes away. I jacked the rear up and let it drive and i could see a woble in the drive shaft. took it off the car and over to a machine shop. they check it out and it supposed to only have at most a thoundsand off instead mine is 4. I have them makeing me a new one. the guy said it would cause vibration but as far as putting it into neutrul and the vibration going away he said it would not be the drive shaft.

what do you guys think.

 

seems rpm related, does it vibrate at the top end of the low gears? could be flywheel/torque converter, maybe a bolt came out.

 

you say the transmission is vibrating visibly and you can see it at the driveshaft?
how is this?
if the car is in park, the driveshaft isnt moving...

i would check your trans mount pad and engine mounts first... extra emphasis on engine mounts

 

Many different variables, the driveshaft being unbalanced would be the big one I'd say, when the trans is in neutral it's taking that load off and just allowing it to free spin. It might be unbalanced enough to just be noticable under load. Had a vibration in mine upwards of 70 mph letting off the gas, (mine was manual though) and found out later that out of the new u-joints I put in one had a sticky cap. One smack of the hammer to it and it went away.

Lol how often is a hammer actually the cure to a problem?

I would say try the new DS with new U-joints first and if it still does it then check all mounting, pull the inspection cover check the flex plate and bolts. THEN if it's STILL doing it and the rest of it is good, check the bearings in the rear axle.

Hope this helps

Edit:
Watch out when you put those turbo 350s into neutral at high speeds. the drum for the planet gears in the trans will shrapnel apart.