Any "casual" translation for 遵命?
Well, there are plenty of "casual" translations. 

In a military setting,


"Yes, Sir."
"Roger that."
"Copy that."
"Affirmative."


*Note that the later three have the same implication as that of 遵命 but is more accurately translated to 收到

In a even less serious setting, 


"Will do."
"Sure."


I will edit when I have thought of more.