Mystery solved. Thanks, internet!

For ages I had been meaning to go to the Tekoi Test Range to check it out. It's not too many hours away from me, and I really wanted to take a day to go there, but never got around to it.

For a long time, it seemed I just couldn't find much info on it other than what I could infer on the CLUI site and the very muddled and poorly written Wikipedia page on it. Maybe my google-fu just sucked at the time, but I honestly don't remember anything showing up when trying to find out more about it roughly 10 years ago. Thankfully, someone on youtube has again beaten me to the punch and made a couple of full videos on it. One video exploring the location and another video explaining not only what was done at the facility in relatively good detail, but also has some actual footage of the tests done there. 

The only thing not mentioned in the video is the variety of the rockets tested there. Some of the more prolific ones other than the Trident SLBMs motors that were tested at the site are likely to be: 

Motors for the MGM-134 Midgetman SICBM
Boosters for the Delta III rocket

The modular Orion booster used on a variety of rockets
RIM-67 Standard SAM missile

and the infamous Pershing II missile

Today the site is supposedly used by a local drone manufacturer for tests, and is listed a potential filming location. 

As an added note that is just barely related, Skull valley is also the same place one of the greatest gunfights in movie history was filmed. I highly suggest you Google search Way of the Gun if you are unfamiliar.