Probably the sheriffs office does not have the resources to investigate a case like this, so they have deferred the investigation to the Texas Rangers. For the Texas Rangers it is likely a cold case, so not much would be done unless new lines of investigation come up. It would sit in a file and be reviewed once a year or so. The evidence they have now is clearly insufficient to move the case forward. and because of the time lapse between her going missing and her body being found, there is not much to look for. The FBI provides logistical support to local authorities in cases like, and that would take the form of specialist forensic analysis, so that would be why they have the remains.
The sheriff technically has jurisdiction, but the state is doing the investigation. Since it is a cold case, both would be directing enquiries to the other, since as far as they were concerned nothing is happening with them and consequently questions should be directed to the other guy (who might be doing something).
If you wanted to find out more of where they stand on the case, ask specific questions, such as if anyone has been cleared or if anyone is a person of interest based on what they know now today. Or something along the lines of "Is SA still considered a POI, or the only POI?". Asking what progress has been made, or how much, will likely get a non-committal answer.
One more point, while the sheriff/FBI/rangers might do the investigation, it is the county prosecutor who would seek a grand jury or file charges, not LE. It might be more productive to direct questions at that office. If they have not been given a file yet, chances are there is little or no evidence regarding what happened to HD.
If those won't talk, then approach the sheriff/prosecutor in CC. They would presumably be involved in some fashion as well.
Raise the issue in the run up to the next sheriff/prosecutor elections, make it a political issue.
There are many things you can do and avenues to push for answers.