Does anyone know what type of locker an L4 is on a 79 bronco rearend

 

Where are you pulling this "L4" from? If you wanting to know what type of differential you have, you need to look at the axle tag. An L denotes a limited slip diff.

For example, 4L10 would mean it has a 4.10 ratio and a limited slip diff.
4 10 would mean it has a 4.10 ratio and an open diff

 

It's from a id chart on fordification.com