Avian leucosis is similar to Marek's. Yes, if this little gal has it, the entire flock carries it. Unlike Marek's, most chickens develop resistance to leucosis and never become symptomatic. The only way to find out if this is leucosis for sure is a necropsy after the chicken dies. Marek's as well, although blood tests for virus DNA can be done, but they aren't 100% conclusive.
If you treat for a crop disorder, and the hen returns to normal, it's then unlikely it's leucosis, although you can't be certain. A lot of crop disorders stem from more serious underlying conditions.