Lots of information about LAX layovers here:

Minimum Connection Transfer Time at Los Angeles (LAX): The Definitive Thread

Long story short, you won't know until the day of the flight which terminal(s) your flights will arrive in and depart from. 1:38 is plenty for a domestic connection anywhere in the US; I generally look for at least 60 minute connections, and will accept lower if I know the connecting airport well and/or there's a good backup option if I miss the connection.