Vibration at high speed

I have a 1971 Chevrolet El Camino, which is based on the Australian Holden HK HG HT pickups.

Up to 110kms /h she rides like a dream, not a rattle or squeak. If I go a little faster from 120 upwards, i'm getting vibration which is coming from the floorboard.

I have new alloys on with adapter plates. So to eliminate the wheels I put the stock wheels back on and same story. I jacked up the rear and put it in drive and got teh speed up to 110km and there was the vibration.

I had the propshaft rebuilt in case that was causing the problem, made no difference, problem was still there.

I replaced the sideshaft thinking that there may be a problem with that but still the vibration.

Any ideas where to look

I'm really stumped with this and it is very frustrating.

Any advise appreciated.

OK - check the driveshaft yolk- @ transmission - try & wiggle it?? if its not secure & does move around -even a little that can cause the symptom you are having= bad sleve @ end of tailshaft area?? also maybe u joints yet you say the shaft has been all rebuilt?? Check the rear end in same area where driveshaft bolts in??

A few suggestions, and these are just suggestions. I have no experience of your car whatsoever.
Gearbox mounts ok?
Engine mounts ok?
Jack each wheel up one by one, and check for run out with a dial gauge.
Wheels balanced properly?

just for my own understanding, this vibration is coming on at 120KM/Hr or 75MPH and above?
I would take a runnout reading on your axel flanges to make sure you do not have a slight bend in an axel.

have you checked front wheel bearings?

possible brake drum or disc imballance

who ballanced your wheels and what how did they do it?

There seems to be light at the end of teh tunnel, or I hope so.

Took my van for on car wheel balancing and man did I almost fall over. They jacked up the right rear and got the wheel spinning and the next thing the entire vehicle is shaking, the wheel is not running true, has a wobble horizonally.
I removed teh side shaft and too to an engineer and the surface where the studs fit was not flat. Gave it a slight skim and then refitted and took her for a drive. There definitely was an improvement.

Now I removed the other shaft and drums and had them skimmed as well. Have not driven the vehicle as yet. I am hoping that after the weekend i'll report that the problem is gone .