Yes, plastic, I believe they make it that way for the purpose of guys like me that forget to take them off. A weak link of sorts. They are well built enough for their intended purpose though.
I just looked at your build and am impressed to say the least!! I do almost none of my own work as I have little talent/time/patience and my buddy's shop can always use the work, at least that's what I keep telling myself.
While looking at your pictures I see you have the antirock in the rear....I was unable to install mine in the rear due to the Teraflex mud flap bracket location. I decided to keep the mud flaps and just keep the stock rear sway bar with longer links. I will add a picture this coming up weekend so you can see.
I saw a comment on brakes on your build.....my 80/60 combination stops excellent w/42's. I have a '16 Power Wagon and '14 Cummins 3500 w/AEV stuff both w/37" and the JK puts them both to shame stopping wise. My JK was weighing in at 6360lb BEFORE the 80/60 and larger tires etc. so 6550+(ish) now. Very impressed with the stopping ability of these Dynatrac units.
I called to order the same BAD wheels you have and didn't order due the fact they were out of the balancing rings.....I did get ahold of Centramatic and got a set of their H1 Hummer balancers and installed them with this set up. Completely sold on this style balancer, very smooth and hassle free.
I will look forward to seeing your completed build!!