To add to what Indy has said, I think you will find that the Gen 1 Part time 4x4 Durangos had the 9.25 inch rearend along with the 4x2 models.
The AWD models had the smaller 8.25 inch unit. This also goes for the Gen 2 models.
Again, the Gen 2 4x2 models also had the 9.25 I think.
As far as the ratio goes, the 3.92 gearset seems to have been put into 33% of the D's while the standard 3.55 was at 66%.
This will explain why the 3.92 is a bit harder to come across.