I have a '97 Ram B2500 Van that needs a new engine since this one has lost compression on cylinders 3 and 6, and a rebuild costs too much money. I was wondering if it matters by the 8th digit of the VIN? (X) or if it matters by the year and the VIN?

 

Depends, are you doing the work or someone else LOL

I would think, if it was a complete engine, that you would want the donor to be the same year. If you are just doing a long block, then 94-97 should work.
There are a lot of 97 (and 94-97s for that matter) in the yards...It shouldn't be an issue finding exactly what you need as long as you look hard enough.