Hi Garret,
We may need to give you a nick name of Turbo ! You have probably been given that name before.
There is a VM diesel group on yahoo that may also be of help to you'
The 2.5 turbo diesel was a 5 speed manual & only in the sport.
If I was considering doing what you want, I would look at a later diesel like a 3.0 CRD.
I dont know if it would fit but a Mercedes OM 602/603/606 may be worth considering. The 602 & 603 have turbos on your side of the pond & are more common than over here.
If that is of interest to you, peachparts.com has a good diesel forum.
The diff ratios in the XJ diesel is different to your one.
They have their own engine control computer.
There is a few threads on here about the XJ 2.5 turbo.
They are much more common in the EU. There are several members on here that live in Europe.
Some of us source parts from Europe from time to time.
Hope that helps.
let us know if we can help any more.
Good Luck !!