Okay, so I had to replace both rear half-shafts on my 318iS (the axles), so I went to the yard. I asked if he had any for a '91 318iS... Bobby at Strictly German said that while the stock 318 half-shafts may work fine on this car, they are actually longer than those on a 325, and since I'm running a medium case differential out of a 325 auto, I should run the appropriate 325 axles.
I wouldn't have believed it myself, but he put two side by side and yup, the 318 half shafts are longer!
Anyways, I went up one weekend and got one from a 325iX, and then another out of a 325i cabrio. When I had them side by side, I realized they two are different lengths!
See picture below. The upper one is out of a '90 325iX, and the bottom one is out of the same year (I think) 325i cabriolet.

Here's a closup of the ends.

For some reason, I guess the iX one is a little longer. So, I need to return it and pull the other one out of the cabrio to get a matching set.