I replaced a 1989 8 3/8 with a 9.25 from a '96 van . The only mods I had to do was a different U joint on the driveshaft to accomidate the different sized yokes.
Just measure the distance between the leaf spring flats. And overall length.
If you are looking for the same ratio, and there is no tag. It is printed on the ring gear itself. Or you can count the teeth on the ring and divide by the number of teeth on the pinion.
As far as I know, any dodge van axle from 71 to 2003 will interchange.
And if you are looking for a junkyard axle, you could luck out and get a good one, or get one louder than the one you seek to replace, which happened to me.