Jennifer Hudson's Engagement Ring Up For Auction!!!

Jennifer Hudson's 10-year engagement to WWE wrestler David Otunga is a thing of the past with lots of hard feelings ... and now the bling is hitting the auction block.

David had Neil Lane design the 5-carat cushion cut diamond ring for Jennifer back in 2008, which has an estimated retail value of $45K.

When they broke up in 2017, Jennifer returned the ring to David. FYI ... in legalese, an engagement ring is a "gift in contemplation of marriage." If the marriage doesn't go through, in most states the person who gave the ring -- in this case David -- gets it back.

So David has now teamed up with IDoNowIDont.com to unload it. We're told David plans to give a portion of the proceeds to a fathers'…