I have a 05' F350 gas with 103k on it. Just had a service done and the tech noticed signs of a possible leaking rear axle on the drivers side.
My questions are:
Difficulty in replacing?
Hidden issues to look for while in there?
Apprx cost of having it done?
If one is leaking should I just do all 4?
Thanks in advance! 
Possible leaks? Special tools are required. You might need park brake shoes depending on the severity of the leak.
All 4, you mean fronts too? If so, no. I might do both rears. You'll want to replace the axle bolts.
Calls for about 2.7 hours to do the job in the book to do the seals.