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Ok I'll post a subject here. 68,4 spd.,10" rear end,drums all around,runs great up until 55-60 mph and then it starts,almost more of a shake than a buzz,only seems to be speed related nothing to do with engine rpm,push the clutch in when it starts and it's still there only disappears when the speed drops.Front end is all new correct Moog parts(wheel bearings,ball joints, tie rods,sway bar end links,idler arm,pitman arm,I-Rock steering box,only original parts are the sway bar and drag link )all torqued to spec and aligned with a Fastrak gauge.New clutch,bronze pilot bearing(had a slightly bad bushing in the trans tailshaft that was replaced)everything else in the trans seemed tight.New u-joints and the drive shaft has been balanced and rechecked twice,trans is up at 2 1/2 deg. and the rear end is down at 2 1/2 deg..Monte Carlo type 7" rallye wheels w/BFG tires 215/60/15 front,Corvette 8" rallye wheels w/BFG tires 245/15/60 rear all 4 have been checked for balance 3 times at different shops,the last 2 said the rims seemed OK for runout on them.New 5 leaf springs on the rear and all the connection points are solid.New shocks at all 4 corners and everything has Energy Suspension poly mounts( front end and drivetrain). I have run the car on jackstands and it doesn't seem to be as bad when it's up in the air with the tires off the ground( I didn't have someone around to look at the rear tires spinning when I had it on stands), what's left to check for a bent axle? One more thing,the engine stands and the crossmember are big block parts,that little bit of an offset isn't a problem is it?

I'm pretty sure someone with more experience will have a better answer, but I'll give you my thoughts for what it's worth. It seems you have eliminated the front end, engine, driveshaft, u-joints & wheels. It has to be a rotating part or the mount for a rotating part. To me that leaves a bad trans mount, mismatched lug nuts, rear brake drum or rotor that is somehow out of balance, or your bent axle idea. I'll be watching to see what the other guys come up with. Good question.

Do you feel the vibration in the steering wheel or the seat of your pants??
This might give us a clue.

The trans mount is new and tight,lug nuts are new and matched.You feel it more in the seat and it increases in strength as the speed of the car goes higher.I haven't gone faster to see if it disappears at a higher speed.

I have always found that a shake or vibration you can feel in the seat of the pants will be from the back end or the drive line. Looks like runout on the wheel was checked but that depends on who checked them and exactly what "seemed ok" really means. Do you have another set of wheels/tires or can you borrow a set that are known to be true and balance and try them to see it the shake changes any?

I do have a set of 14" wheels but they are a mix of offsets,don't know if that would make a difference or not if I were to try them.

First thing I'd try is swapping tires and wheels front to rear, see if it feels any different.
As long as you're not vauling curbs or autocrossing, the 245's should be ok in the front.

I agree with the above that its probably in the rear of the car but not in the driveline as you say it does it in neutral at speed.
If you don't want to change the tires & wheels try pumping the rear tires up to 45 lbs and see if that makes a difference.
If it diminishes it's probably tire runout,belt separation etc. problem.
You CAN balance a tire that has radial runout & it will give you a vibration.

The 245's almost work on the front,a little rub at almost full lock,it's lowered a little in front from stock,I think the backspacing kicks them out a little too far but I could drive it a little. I'm really just fishing for ideas right now,there is no way the Camaro is going outside with all of this white crap on the ground.I got tired of trying to figure this out last summer , it kind of limited how far I could drive comfortably and I don't want to damage anything else with the shaking.It's the only real problem that I have except for a small lean(pass. side high) and a few small oil leaks( old motor).

After check the tires/rims with a known vibration free set, I would jump to the rear drums and see if they are true. You should have noticed a pulse type vibration when you put the brakes on. Also, applying the emergency brake slightly while going down the road should enhance the vibration if the drums are out of round.

Kev