It was like a towing package. A metric tonne is larger than an imperial ton, so hence the larger axle.
Now what to look for, there are tons of links around for axle IDs showing the different pumpkin shapes, print one off for the D44. I believe the most common years for the XJ D44 was 87, and up here in Canada I heard we got more than the US.
Other than that I don't know anymore, happy hunting