From the joint?
It is possible that one could post to RXMuscle while not being able to post on Getbig. Getbig sometimes comes up blocked supposedly because of the porn related material. Not having experience with prison, I don't know this for sure, but they might block certain "adult" sites while allowing prisoners to post on other sites.
Is Groink housed in a county jail or a state prison? In Oregon, some offenses will land you in County while more serious offenses with longer sentences require one be in a state prison.