Pac rapped over the jr mafia beat on hitemup cuz he was clownin on them, not because he couldn't get his own beats. I don't know if 2pac did toss it up before or after that blackstreet track came out because they dropped the same year but if he sample the beat it's probably because he, suge, and the rest of death row didn't like dre. Rappers usually jack other rappers beats when they wanna clown on them.