There are several possible causes of bark peeling on your lilacs. The most likely cause is squirrels and/or rabbits munching on the bark. This can be alleviated by enclosing the shrubs in a wire mesh. 

Rapid freeze and thaw cycles sometimes cause splitting and peeling bark on lilacs. This often happens at the site of a previous injury. Sun scald is another cause especially on the south side of the shrubs.

Once the bark is stripped off and the cambium is exposed it is a wait and see situation. Your shrubs may be fine - or not. When critters do this kind of damage the shrub may appear to leaf out just fine in the spring but as summer stress (drought and heat) happen the shrub may die back. 

If parts of the shrub survive and leaf out you can do some pruning of dead branches. Pruning is not something I would do until late spring. Also, keep them well watered but I do not recommend fertilizing. If the plant is not able to move water and nutrients because the protective bark is removed then fertilizer will not be helpful.

These links will give you some additional information:

https://homeguides.sfgate.com/squirrel-damage-lilacs-71907.html

https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-bark-coming-off.htm

Thanks much. We have an excellent population of both squirrels and rabbits.

My pleasure.