This is in reference to a 13 SEER, 5 ton, RHEEM a/c unit:
There is a difference of approximately 100 psi between the suction and liquid lines when the system is off.
What is this lack of equilibrium an indication of?

Some systems have a hard shut off thermostatic expansion valve, They do this for efficiency. A system with this device will not equalize during the off cycle. Good luck. Mike