If it's slipped tendon, you would be able to feel to tendon out of the groove and you should be able to push it back into place, but it will probably pop back out.
This I found these interesting:
http://www.peacockemporium.net/Slipped_Tendon_In_Peafowl.html...
X2
This is a cut and paste from one of my other replies. Maybe it will help you.
FWIW, the pathologist that I work with says that Merek's is one of the leading causes of death that they see in chickens at their lab. He also said that it's an old wives tale about it only affecting young ones...