With the car on axle stands and when the gear box is in neutral, there is movement up and down of the prop shaft as it comes out of the gearbox.
Is this right ? There is a vibration that comes in at 85mph and gets worse with more speed and feels rubbish at 115mph !!
Also, when shifting from neutral to drive , there's a lag of maybe a second. Is that normal ?
And when shifting from OD to 3rd and 3rd to 2nd, there's a lag.
Are these signs of an old gearbox. The car has 139k miles.

 

there will be a pause when downshifting. check u joints and rearend for play. also while your at it have a look at your bushings and the trans mount.

 

As for the tail shaft it depends on the amount of play. The slip yoke just slides into a bushing. It is simple enough to fix. The shifting issue you talk about could be normal depending on the delay. They could also indicate your trans needs a service. Old fluid can cause soft/delayed shifts. So can low line pressure and low governor pressure. A heath check would include pressure tests and service maybe even a flush. Once you get the pan off and you can see what is in there you will have a better idea how far gone the transmission is.

IMO the easiest way to track down vibrations is to get vibration software for your smart phone. Get the harmonics reading and convert that to cycles per minute. at a given speed then use your Tire Diameter and rear ratio and calculate what is turning at that RPM. If the cycles per minute match the RPMs of the drive shaft then you are on the right track.

In my experience drive shaft vibration tends to go away off throttle. Vibrations that just keep getting worse tend to be a bad bearing, bent something or a balance issues.

 

Thanks Gorn, will follow up on the vibration app path.
Been plotting trans pan drop and swap, must be time for some action there.
You're a very helpful chap Gorn, thanks again.