Darn I hate it when the simple obvious thing is not the cause but in my defense that's what had happened when I lost all 4-wheel and the fix was a less that $1 bushing from DCJ.
Just to be sure I assumed that by "oil" levels being ok you mean the ATF fluid level was ok. Possible stupid question but I did lose all tranny motion on the fly in my Dodge D250 Cummins when I lost a bronze ATF coolant line fitting and the tranny pumped all the ATF out in like 2 miles. No long term damage but rather more than a bit of a surprise.
I'm not at all an automatic transmission person although I have redone manual transmissions. That said the 545RFE is a pretty common DCJ transmission (e.g. not limited to the CRD) so you may have to find a good independent transmission shop although how common that is in your location I don't know. The 2 things that come to mind are 1) the fluid pump may have failed as the original design had some issues and IIRC has been replaced with a beefer model or 2) there is a plate on the side of the valve body that has been know to come adrift due to bolt failure or backing out which I suspect would stop the tranny from working at all. The latter can be diagnosed by simply dropping the tranny pan and looking at the valve body and the fix is to install the TransGo kit (without the resistor that they used to include). The TransGo kit is a relatively easy DIY I understand although I had it done as part of SunCoast installing my TC. See
http://liberty.eurekaboy.com/transgo.htm for an older post on the topic and a picture of the valve body and plate I mentioned.
Beyond this I'm out of my league.