Final answer:
Quickly conduct a secondary assessment.
Step-by-step explanation:
If you detect an unstable pelvis with crepitation in a patient complaining of hip pain, the appropriate action is to quickly conduct a secondary assessment. This is necessary to further evaluate the patient's condition and determine the best course of action.
Applying a traction splint or increasing the flow of oxygen are not appropriate actions in this scenario, as they do not address the specific issue of an unstable pelvis with crepitation. Rechecking the vital signs may be done as part of the secondary assessment.