The schematics are exactly the problem. I have a 93 on the driveway right now and, determined to figure out the wiring, I spent about an hour with a meter Friday. The problem is that there are errors in the 93 schematic at the cluster, so having the book probably won't help you. As you may have found, there are four inputs to the speedo head right side board on the ABS clusters as opposed to three on the earlier electro-speedos. The only thing I've determined so far is that instead of power being plumbed into the rest of the cluster via the three prongs (from the L connector) on the back circuit, one of the four on the ABS cluster is bringing power into the speedo head. Obviously something to do with the ABS ECU, but I haven't figured out what it's doing yet. Putting the older speedo head into the cluster won't do the trick. I see someone else posted to remove some of the teeth on the tone ring. Not a bad idea if it can be done through the sender access. Taking apart the diff is obviously what you wanted to avoid to begin with! Wish I could be more help, but the cluster part of this riddle is a very tough one to figure out.
DS