This error comes up when I get my 9wd frame as well. I have also attched the log files when the error occured.
I apologize for taking so long, but I finally figured it out. Log's didn't help at all, and I had to dig DEEP for this one... Code: ["multiShaft", "diff", "rearDriveshaft", 1, {"gearRatio":10.33, "numberOfOutputPorts":4}], "numberOfOutputPorts" is what you need to make it work. Since multiShaft can have as many outputs as you wish, it needs to have a defined number of outputs so that the game knows how many to let it do the math for. Edit: Also, you don't have to use that code exactly, I was just screwing around and I finally got it to work, so I just posed what I had from the vehicle I was messing with. The number of outputs is the part that's important.
I found a super old post from like...2015? And in there he was trying to move over to using more realistic code for his mod, but it wasn't working so he posted it asking what he was doing wrong... but commented out was his code he was using before and it had the miltiShaft code in it.