Sqeaking When In Gear

[sledgehammer1990]

For the past few months, I have had a squeaking sound coming from underneath my explorer. It is only when I am driving it squeaks and when I throw it into neutral, it goes away. Any ideas on what could be causing this?

 

[Robrt32]

first guess would be U joints

 

[sledgehammer1990]

Thats what I thought. How many do I need to replace? How many are there? Which one do you think needs replacing?