Mopreme Shakur -- Tupac's stepbrother who rapped alongside him in the group Thug Life -- stopped by "Piers Morgan Uncensored" Friday ... and Piers asked him about the well-known theory that the Bad Boy founder played a part in 2Pac's death.

Piers mentions how Diddy actually called Mopreme back in 2008 and denied any involvement in his little brother's death, after an infamous L.A. Times piece alleged associates of the rap mogul planned the 1994 shooting of 2Pac at Quad Recording Studios in NYC.

Worth noting ... the piece alleged Diddy's associates were involved in the '94 shooting that injured Shakur, not the 1996 shooting that ultimately claimed his life. What's more ... the Times famously retracted the article just weeks after being published ... 'cause they no longer believed their own reporting to be credible.

Mopreme, however, still isn't convinced Diddy's hands are clean when it comes to his brother's death ... saying he told Diddy back in the day the truth of who did it still has not come out -- so, they needed to wait and see.

As we reported ... prosecutors in the case against Keefe D -- the man they arrested last year in connection to the murder -- tried to use statements he allegedly once made -- claiming Diddy implied he'd pay someone to kill 2Pac -- in an attempt to get a judge to deny Keefe D bail.

After Keefe's claims resurfaced, sources with direct knowledge told us members of Tupac's family retained Alex Spiro and Christopher Clore to look into the allegations.

Diddy has denied all wrongdoing in 2Pac's death multiple times over the years. He has never been arrested or charged in connection to Pac's death, and law enforcement sources told us he's never been a suspect in the case.

Still, sounds like Mopreme's still not buying Diddy's claims ... choosing to wait and see instead.