Apparently it's not as uncommon I had thought. Just a year before a Tennessee state judge was charged with obstructing justice in an FBI investigation. But the FBI was investigating him (not some other cause), basically for corruption.
The FBI on Jan. 25 opened a probe of whether the judge [Moreland] extorted and accepted sexual favors, travel and lodging from people with whom he had close personal relationships, according to the complaint. [...]
Moreland then tried to obstruct the investigation by attempting to pay off one witness with $6,100 so she would recant previous statements made against the judge, the U.S. Attorney’s Office said.
The judge also tried to arrange to have drugs planted on the witness and orchestrate a traffic stop so the drugs would be found, destroying that person’s credibility, the U.S. Attorney’s Office said.
I suspect most such cases of obstruction of justice by a judge would be like this, i.e. in the course of an investigation in the judge's (prior) activities.
Still, if someone can provide a more comprehensive anwer... I'll accept that instead.
