K48 - jackstall is where the aerodynamic backloads from the blades (specifically on the retreating side during manoeuvering) overpower the hydraulic jacks because they are not man enough for the job. Aerospatiale (as they were then) claimed this was designed into the system for Gazelle but most people are of the opinion that they just under-engineered the hyd system.
I have heard of, but not experienced, similar problems in Squirrels and another label that was given to the condition was 'control transparency'.
However, a hydraulic lock is a hydraulic lock with pressure trapped on both sides of the actuator, usually due to a blocked port or failed pilot valve. Only a detailed stripdown of the components would reveal the source of the problem and even then might not be 100% conclusive.