I had the SB-19-39-001 fix on my 2017 model X in August 2019. I recently raised my suspension to test the shudder, and yes it is back, but not as bad as it once was. As I compare my 2019 invoice to SB-21-39-001, it looks like the RH and Clevis assembly part numbers are the same but there is a new part number for the left hand half-shaft.
I am curious about what is known about the difference in this fix (they are just part numbers to me) and why it might be a more permanent fix? Or is the finality of this fix just hopeful thinking?
There's a few different things. The shaft and clevis are different. There's also a break-in period that seems to be new. From all the reports we've heard so far with the new parts installed, THIS is the permanent fix.
I've also heard from someone who'd be in a position to know that this should be the one. (In other words - complained about my shudder, the response was 'don't bother yet'... now that the new parts are available, it was 'NOW bother with getting it fixed.')