Definitely rootstock with the trifoliate "leaves of three". I agree with fruity paws, you do have some scion left in the third pic and it's flowering so it's still alive. Although it could be flowering because it's dieing and sending out flowers in its last ditch effort to reproduce. If it were me instead of cutting the rootstock portion back and waiting for the scion to grow back I'd pull the tree and start over with a new grafted tree and this time keep an eye out for rootstock suckers before they get this large but I'm an impatient gardener. Just my .02.

Evan

Oy. This will be a radical prune for this novice. But I intend to do just that. Because the sucker looks to be above the graft line, and the fruiting wood below that, it remains a curiosity as to how it slipped by me. There's an enormous grapefruit tree, and dwarf orange next to it. Thanks for identifying the nightmare for me.

You can try to air layer the fruiting branch and reduce the big tree to two lower branches at the same time ( in the warmer months). If you're not in a hurry off cause.

the small fruiting branch is too slight to propagate. will just prune and see how the modest limb does this year.

the sucker looks to be above the graft line, and the fruiting wood below

No, the red line in the photo below is the original graft. The blue line shows where you will have to prune.

Thanks. Pretty sure the tree came to me with this enormous sucker. Lobbed it off. Hoping that will not only redirect energy, but spur some new growth.

I would remove or loosen that plant label. Looks to be strangling the trunk. After you lob off the aggressive root stock growth, you might try carefully staking the remainder of the scion into a more upright position.

You'll probably find that removing such a large section will also spur new rootstock shoots. You need to watch out for these and rub them off when really tiny. Otherwise they will grow very rapidly and you will end up repeating the problem!