Did I install this trailer leveling rod/linkage correctly?

It's attached correctly but it probably needs some adjustment. It looks like you have too much air in the bags the way they are now.

 

Judging ftom those lines in the plastic base, I'd say your about 2" too high. Other than that, looks good

 

Judging from the wear, yes. You can see the lines in the rubber have worn the same lines into the plastic base. That is where it normally has ridden. Doesn't mean it was correct before, but it looks pretty close to where it should be.

 

Gotcha. You're not going to get a vibration from the trailer suspension height. You can definitely get a rough ride though.
Nonetheless, I'd still say those bags are roughly 2" too high. But there is a spec you could probably find on their website, or just call customer service

 

The procedure is on the Hendrickson site, but it won’t let me link to it.
Basically measure top of bag closest to leveling valve to the axle, then add half the diameter of the axle.