ON the eve of their wed­ding, JENNIFER ANISTON and JUSTIN THEROUX traded insults in an ugly blowup that nearly ended their plans to say “I do,” sources told The ENQUIRER.

The nasty love spat exploded shortly after they arrived in Cabo San Lucas, Mexico, on Dec. 23, but ended happily with the couple back in each other’s arms, say insiders.

“Cabo is Jen’s favorite place on earth, and she wanted an idyllic, relaxing getaway with Justin,” an in­sider told The ENQUIRER.

“Instead, they got an angst-ridden nightmare.”

The holiday got off to a great start with the lovebirds, who became engaged on Justin’s 41st birthday in August, putting the finishing touches on their wedding plans while loung­ing in the sun, said the source.

“But things went horribly awry when Justin suddenly turned intro­spective,” divulged the source. “He started to pick apart their life togeth­er and their plans for raising a family around Jen’s booming career.”

That prompted the former “Friends” star – who’s often intimi­dated by screenwriter/actor Justin’s intellectual side – to point out that she felt Justin overthought things and could benefit from being more spontaneous, continued the source.

“After that, their conversation de­teriorated into a shouting match,” the source said.

“Justin accused Jen of living the shallow life of a Hollywood screen queen.

“Jen responded by saying that Justin had no feel for her inner self and called him a pseudo-intellectual who didn’t know the meaning of the word fun. Then she burst into tears and huffed off.”

The confrontation had apparently been brewing for weeks as the two struggled to overcome their dramatic differences.

“Justin worries that he and Jen are not on the same intellectual plane and often complains that he feels smothered by her Hollywood fame,” revealed the source. “He misses his artist and musician friends in New York City and has said many times that he’d love to move back there.”

Justin also dislikes being used as 43-year-old Jen’s pawn in her running feud with ex-hubby Brad Pitt and his fiancée Angelina Jolie, according to the source.

“Justin feels as if he’s Jen’s prop to prove to the world that she’s finally found love,” explained the source. “He feels that on some level their relationship is almost a sham.”

Adding to the couple’s stress is the frail condition of Jen’s 76-year-old mom Nancy, with whom she’s long had a stormy relationship. After suffering a stroke in 2011 and falling twice in No­vember, Nancy now might be developing Alzheimer’s disease, as The ENQUIRER recently reported.

But, happily for Jen and Justin, love seems to have won out in the end.

“After a couple hours, they each realized how foolish the fight was,” said the source.

“They both apologized and re-pledged their undying devotion. However, for a while it looked as if they were not only going to call off the wedding, but the engagement too!”