Diagram of the diff locks?

[John Larson]

You wrote: 

If they both worked fine before, then most probably the "mechanic" 
switched hose positions or worse on the middle diff. Reverse them and 
see, and inspect for brocken actuator hose connections."

HTH, Ado

Another possibility is a broken actuator hose nipple.  The fit is tight in there, and the actuator is easily damaged while muscling the trans out and in.  I've seen it more than once.  John