If you recall in the rule book on page 22 in the striking resolution example Darth Vader had a focus token placed on him during combat so he wasnt able to attack.

My question is can you target a unit that becomes exhausted during combat if it was a participating unit? So instead of Luke dealing his 2 damage to the Kuati Security Team could he have instead dealt that to Vader?

Short answer: yes. Long answer: being focused does not remove a unit from an engagement. Also, Luke's ability (targeted strike) would have let him damage Vader even if Vader wasn't participating. The reason to hit the security team is to kill them before they strike back.

ok awesome, that really makes much more sense when i have remaining units that can only do damage to units