Yes, Lanka is mentioned in the Mahabharata. Sage Narada came and told Dharmaraja Yudhishthira to do the Rajasuya Sacrifice because his father, King Pandu, was in the Yamaloka and wanted to go to Svargaloka. Panduraja in his original form was a Marut devata by the name Paraavara Marut. One day he failed to offer his respects to Indra while Indra was coming into the court, and Indra cursed him to be born as a human and never return to his original form and be in the Yamaloka. Paraavara Marut asked for the remedy of the curse, to which Indra said, ‘In future, Yudhishthira will do the Rajasuya Sacrifice, and then you will be freed from the curse and will merge in your original form.’
84) “In his original form, your father was once performing a penance. Indra, the best amongst the Maruts, came near him. Your father, however, did not stand up in respect. Hence he cursed him”.
85) “You shall obtain the birth of a human. After your death, you will return to the Deva loka. However, you will not unite with your original form, and hence Harishchandra will be greater than you then”.
86) “When Paravaha (the original form of Pandu) apologized, Indra told him – ‘If you perform the Rajasuya, you shall attain your original form again and become superior to Harishchandra'”
87) “Therefore O Dharmaraja! You should certainly perform this auspicious task. All of you too shall obtain complete merit as per your own qualifications”.
[Mahabharata Tatparya Nirnaya, Chapter 21, Pandava Vanapravesha]
And that’s why here Sage Naarada asked Yudhishthira to do the Rajasuya sacrifice. Thus, Yudhishthira sent his brothers for digvijaya. Out of those people, Sahadeva went for the digvijaya in the southern region. Because for the Rajasuya sacrifice, we need to bring taxes from all the kings from all over the world. So, Sahadeva was aiming for Lanka. He remembered Bheemasena’s son Ghatotkacha, who came there immediately and, as per Sahadeva’s instructions, flew to Lanka. There he met Vibheeshana, who was still ruling Lanka. Vibheeshana was extremely pleased by the humility of Ghatotkacha and gave the tax to him for the Rajasuya sacrifice.
213) Sahadeva sent Ghatotkacha, who came there upon mere remembrance, to Vibhishana. He too gave a lot of tax to Sahadeva.
214) Vibhishana remembered all that Sri Rama had told him. He realized that Sri Krishna was Rama Himself. He also observed the strength of Bhima and Arjuna, and that the Pandavas were all Devatas. Hence he gave the tax.
215) The son of Bhima came back to Sahadeva with one mahougha of precious stones. Sahadeva returned to his city. Note: One mahougha is a crore followed by sixty zeroes!
[Mahabharata Tatparya Nirnaya, Chapter 21, Pandava Vanapravesha]
This is as per the Tatparya Nirnaya, which SriMadhvacharya does after reading the Vyasa Mahabharata present all over India, correcting the interpolations (as He Himself mentions in the MBTN ‘Kvachid Granthaaha Prakshipanti') and mentioning the correct Nirnaya for the Mahabharata as per Lord Vedavyasa. Even the Daakshinaatya Paatha of the Mahabharata mentions this.
Actually, after killing Raavana, Ramachandra Himself instructed Vibheeshana that in the future, Yudhishthira will do the Rajasuya sacrifice, and you have to agree to it and give the tax. So yes, Lanka is mentioned in the Mahabharata. But one must note that this Lanka is not the SriLanka which we see now. This SriLanka is the Simhala Dvipa and not the Lanka where Ravana lived, and Vibheeshana is still ruling. It is an island like the Andaman and Nicobar Islands, Lakshadweep, and the Maldives. The real Lanka is there, but is invisible to our eyes. That’s why Lanka is an invincible city, where Vibheeshana rules and worships Ramachandra.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!