The stock diagram shows that little middle "retainer" washer right under the top plate ("suspension support"). That comes as part of the stock top plate assembly. I believe it is glued in. For 5100s, that part is not used. However, if you ever change to, say, OME Nitrochargers, you will need it again. So save that thing when you take it off. It is special order from the dealer.
That retainer is a specific diameter that needs to fit precisely around the shock piston shaft and eliminate any play.
Look under your new, stock top plates, and you should see it pressed on there. See if the 5100 shaft even fits through it. I think you will find that is does not, and you need to pull the retainer off the bottom of the top plate.
if I sound a little unsure, it is because I had my front 5100s done at a shop because I had no time, so I never R&Rd them myself. Later, when I decided to change to Nitrochargers, I removed them from my truck and took them to the same shop with their big coil compressor and hung out while the tech did it. The OME instructions specifically say to resuse it. Well I was screwed, because they were gone, and special order. The OME shafts had play without it.