I took the duster out for a rip today. I put it to the floor and run thru the gears til about 70. I let off and coasted to the stop sign. About 50 mph it started a real bad vibration then stopped around 35 or so, I'm going by feel no spedo in the car...

I went across the intersection and drove to the church to turn around kept it around 35-40 and not a sound. Turned around came back to the stop sign and still not a sound. Left the stop sign was in first gear and hit it went thru the gears to about 70 again and let off and grabbed neutral. Not a sound til about 60 and stopped around 35-40 again...

I'm confused on what it could be

Anyone ever experience this? When driving under 40 mph it doesn't make a sound...

what kind of drive shaft do you have? Those drive line vibrations can be a pain to figure out.

It could be a number of things, driveshaft/ujoint/flywheel etc.

One thing for sure is if you feel a vibration thats strong enough to feel. You have something major going wrong.

My first question is what is what has been changed on the car since the last time you had it out.

I have a Dynotech 4" CM .083 1350 u joints.

It was in neutral when it was vibrating. I rev'd the engine and no cchange. It also only vibrated on coast.

Only thing that has changed was a rear brake bracket was cut off and a new one welded on.

Check the pinion nut.

I experienced a similar problem a couple of years ago. After leaving the line and at 87 mph I felt a sudden growling vibration that stopped when I let off and coasted to bout 75 mph. It did it again the next run at the same mph. Driving to the track and back was no problem, even on the interstate at about 75 mph. I changed out the pumpkin and let it sit til last week when trying to figure out if the noise was due a bad setup or runout. Well when rotating the gearset I noticed that the pinion nut was loose. Under closer scrutiny, I saw it was stripped. The pinion shaft looked fine so I replaced the nut and tightened it to get about 7 in/lb resistance to rotation. I will put it back in in a couple of weeks. Hope that new nut fixes it.