The MLB legend stripped down to his undies as part of a partnership with underwear company Saxx -- filming a 75-second commercial with the goal of raising awareness for testicular cancer in mind.

In the vid, Rodriguez rocked a pair of boxer briefs that he insisted held his family jewels "like a catcher's mitt" ... before he introduced a "Ballgorithm" that will ultimately raise cash for the good cause.

Rodriguez and his co-host explained ... the algorithm -- a math equation centered around the number of bad balls that a pitcher hurls during a contest -- will be applied to every Rangers vs. Diamondbacks World Series game this fall ... and then that figure will be multiplied by 1,000.

That total number will then be turned into dollars and donated to the Testicular Cancer Foundation -- a cause Pudge tells TMZ Sports he cares deeply about.

"The purpose is just to make people laugh and make people aware to go check themselves," he said, "and we did a nice commercial."

The MLB Hall of Famer -- who's now working as a special assistant to the general manager of the Rangers -- said he's been catching a lot of ribbin' both inside and outside of the Texas clubhouse over the vid ... but he couldn't be happier that it might prevent tragedies for people going forward.

"For me, that's the most important thing," he said. "Pay attention. This is a disease you can get in the early ages. So, our message is just to go check, because you never know."