Heres a pic of the setup in my Beaver NC5 mill, the line driver is the board I made up to convert the single ended step dir to differential, I made it so that it hooks directly into the PMDX breakout without wires.
Hood

Hood
   Are you using an a negative voltage supply with your single end to differential Driver?
I have some chips here that have 55mv to 4.95v single to different on a single 5v supply but 0 to 4.95v with a negative supply.
What are the Chips you are using with your line driver?
I have on hand some LT6450 from Linear Technology but they only have on channel input.
I was just going to use short shielded cable to tie mine together but I am open to try anything to help with reduce noise.
I will look into the PMDX boards thanks again for the help.

The drivers I am using are SN75172N  http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=630-875

The way I look at it is the DSD drives can accept differential input so its best to supply it. I know quite a few people that have tried the drives recently with single ended inputs and have had problems, once they made up a line driver circuit they disapeared.
Hood

Hood
   Do you have a schematic for your interface or do you use a particular on from the app note?

I should have a drawing somewhere for the PCB I made if thats what you are wanting?
Hood

Do you have the Circuit diagram for it?
Yes the PCB layout would be great to have.
Thanks again

I just downloaded the pdf from RS and used that for connection diagram.
Have looked here and cant find the drawing or code but I have given it to a few people so I must have it somewhere, it should be at the workshop, I will look tomorrow.
Hood

Hood
   Do you use any type of balance resistor between your Diff signals?

Nope, not an electronics guy and I think someone once mentioned a resistor may be required but as it had been running happily for 6 months without issue I didnt bother.
Hood

I just read your Balding Beaver thread and I like the servo for the spindle you are using that is what I plan to use on mine I think I will use a 5kw motor for my spindle it has a 7 HP motor now. 
As far as the balance resistor I believe the reason we use them on RS485 communication is that is balances the load out over the long wire length.

I have found some 1398-009 drive amps and some J-4050 motors which are I believe overkill but are a good price the motors have 6.9nm of torque and 4000 rpm max speed.
I also have found a 1398-030 drive that would be nice for the spindle motor but have not found any motors for the spindle yet.