I have the driveshafts from my XJ LS project car and my XJR parts car. Going to take one along with Camaro driveshaft to machinist to make one for for the project car.
The XJR is longer because the Mercedes transmission is shorter. Since they use the same rear end, I assume the strength of the driveshafts are the same and it does not matter which one I use. Correct?

 

I would presume they are the same material/strength. The shop I worked with felt a one piece would be the way to go, so we made a custom one using Sonnax billet adapters at either end with a little modification at the rear one to fit the Jag diff. Can provide more info if you wish.

 

That is an option that I’ve considered. Much more expensive.

 

It was between $700 and $800 for reference.

EDIT: You could use a flange adapter plate to bolt a conventional u-joint yoke to as opposed to a complete billet one piece yoke like I did. You'd drop the cost significantly using standard yokes.