I'm wiring a 100amp subpanel for a detached garage. I want to place a 100amp breaker in the house breaker panel to feed the garage. Will #2 aluminum direct burial cable w/ 2 conductors and the shield as the neutral work? If not can I run that cable with a separate neutral or ground wire to the subpanel?

A "subpanel" needs 4 wire feeder, two hots, a neutral, and an equipment ground conductor. If you want to direct bury cable use #2- w ire URD cable.