If you can get rabbits that weigh the same as the jumbo rats you're feeding now I would urge you to switch both of them. Rabbits have heavier bones, more lean muscle, and much less fat than similarly-sized rats, and jumbo rats in particular are often old, retired breeders with a high fat content. I've also noticed mine seem to be satisfied longer with a rabbit than a jumbo rat of the same weight, I'm guessing because it takes longer for those thicker bones to get digested.
Rabbits do have some fat - about 10% - enough to encourage the boa to slowly process extra fat they're retaining without the massive dump associated with suddenly putting a critter with fatty liver disease onto a low fat diet (this is more of an issue with cats IIRC).