Sirion Rally IV 4WD not working


Hello everyone, I am a very happy Sirion Rally IV owner and just joined the forum. My question is about the 4WD and transfer box? does anyone know how to prove its working, and if its not working, how to test or diagnose the fault.

I have tried the obvious, (raised car on 4 axle stands, and then run the engine, the rear wheels never move!!) tried holding one of the front wheel still to engage the Diff, no difference!

I had been advised to change the oil in the transfer case, did that,Oil seemed VERY black and dirty, so had hoped to make it better, but further on road tests on Grass and snow, did not show a major improvement.

Are there any other ideas out there on what to do to test its working? Also does any one have an understanding of how it is supposed to work?

Many thanks to the forum for your suggestions!