If enemy unit dies with a reaction on my Defiant Dance, can I choose another unit for -2?

No, you cannot choose a new target for the -2 Might effect if your original target dies before the spell resolves.

According to Rule 352.14, when you play a spell, you must select all required targets at the time of casting. Once those targets are locked in, they cannot be changed.

If a target becomes invalid (for example, if it leaves the battlefield) before the spell resolves, the spell will still attempt to resolve as much as possible. However, you cannot "re-target" the effect to a different unit. If the unit you chose to receive the -2 Might is no longer on the board when Defiant Dance resolves, that portion of the effect simply fails to occur. The +2 Might effect on your other chosen unit will still resolve normally.