You'll have to lose the cops, but once you do (or call Lester to get rid of them) McReary will ask you to bring him back to a safehouse in Los Santos.
Kill the drivers and steal the van to save McReary. This is marking a police van, escorting McReary to jail unless you intervene.
While playing the game, keep an eye on the map for a small, blue dot appearing close by. Packie McReary is unlocked in a random event while playing in the open world of GTA: Online.