Use your anatomy lessons, Weedhopper.
1. What anatomic structures which you use daily run through the area of pain? Or can it be replicated by pressing on nerve plexi?
2. Try to replicate the sensation, probably using resistive contraction (i.e., re-enact lifting a patient by lifting the bumper of your truck etc).
3. Ask yourself what makes it worse and what makes it better?
Lastly, if it's work related, tell your boss. Maybe she or he has an answer, and it may be covered under work comp, or its an ergonomics issue they need to fix for everyone.
PS: Do you raise the ambulance litter with an underhand bicep curl? If so, try not to actually curl in (palmarflex) the wrist, nor let it fall back (hyperdorsiflex) under load, try to keep it neutral with whole body coordinated lifting.