96 Astro van replace rear end?Model yrs compatible

No it is not OK any year. 1) your van - do you have abs brakes. 2) Multi leaf springs or single carbon fibre leaf spring.
These 2 points are very important.
The axel to leaf mounts are not the same. The leaf to the frame rail is also different. Fabricating or shimming might not be overly safe in a heavy braking situation or stabibility of the van overall.

Ensure that the axel you buy is either a multi leaf or a single carbon fibre leaf, whichever your original is.
BE SURE NOT TO BANG, SCRATCH OR GOUGE CARBON FIBRE LEAFS.
ABS brakes - if you currently have abs then you should ensure that the complete axel you get also has abs sensors and wires. Non abs axels will work but your abs dash light will always be on and you will notice a huge differance in brake performance on all 4 corners.