73 Duster, rebuilt everything nearly, balanced motor, shift- kitted 904 thats been beefed, recut and balanced driveshaft, rebuilt rear, as is bearings and seals. New Cragar wheels fresh balanced, Poly trans and Motor mounts.

Issue: at about 55-60 I have a "feel it in the lower back against the seat buzzy vibration..someone of a fluxuation. I'f I ease into 65-70 the feeling increases some but I detect towards rear of car a steady, somewhat up-tempo flux with the vibration. the lower portion (not the top mind you) gets a little 'buzzy'..any more than 70 and the lower have of the dash is shaking rather badly. seems most of the problems are at that belt line of the car. kinda odd.

I have a feeling it's the axles because at 5-20 what I have a sorta "bumpiness" to the ride as if the springs are to tight but I don't think it is, even though they are new HD..it's a bump bump bump.....bump bump bump pattern. But I'd like input, first time I've really had it up to speed for any rate of time. Rear is 8 1/4 and the axles were never checked or anything. Everything else is pretty fresh, whats it seem like first?
thanks

put the car up in the air and have a buddy accelerate to those speeds while you are behind it. Look at the wheels and see if they're wobbling. I'm in a similar situation right now.

I could visibly see the wheels wobbling while the car was in the air. Gonna pull the axles tomorrow and have them checked out.

Have you checked your pinion angle though?

No on the pinion angle, I have 2" over springs on the car though, But I had that same set up with a Challenger and never had the vibration.

I have bought new tires and found one to have separated belts and you could see it as the car rolled while someone was driving behind me. I have also had the tire change place bend my rallye wheel before. Found them banging on it to straighten it when I arrived to pick the tires up. It had a slight wave to it when it rolled as well.

I'll get rear up in the air this afternoon sometime and have a look when my buddy comes over. Since the vibration is so heavy and obvious at a somewhat lower speed, is that why it seems more likely to you guys to be something I can see in the wheel/driveshaft, etc?

When you installed the springs did you do the final tighten with the weight of the car on them? If you put heavier springs on you might have changed the pinion angle, you stated it has a bump, bump, bump replace that with bind, bind, bind. It could be a pinion angle problem. If you put it in the air and run it I would suggest jack stands under the axle as opposed to a lift so the weight of the car is where it should be to simulate ride height.

Food for thought, try checking the position of the rear spring shackles in conjunction with the pinion angle. Also your shock rates.
May be worth the effort.

I had that vibration before and it was the driveshaft, have it rebalanced

put car in air, jack stands under the diff. I put car in gear. my driver wheel was straight at an "idle" spin, and when i ran it up to about 2000-2500 rpm. the passenger side was another story, it would wobble at the idle spin, and it got bad the more i went, the center cap was noticeably off center too. my friends dad was an ASE mechanic for 35 years, he looked at it during all this and said it has to be the axle with it moving that much at the hub. Later on I thought I'd like to be sure, would it be "safe" to rub in idle with wheels off and lugnuts on to hold the drums? I'd like to see if it's obvious there.

thanks for the help.

oh, when i tightened suspension, I put the shocks, springs, and axle in just enough to hold, dropped car down on it's weight and tightened everything. I was aware that was important on the front end parts and applied that to the rear of car as well.

yeah i sorta made that mistake today, luckily I was at a very low rpm. i was quick, heard a click and realized it needed to then go to neutral