I have 3.73 gears in the back. Not sure what kind.

When I let off he gas they sing. Is this a sign of them going bad or getting weaker?

 

Most aftermarket gears (your's are automatically aftermarket being they are 3.73) make a whining noise either on acceleration or deceleration. It's normal, but it is also a good idea to check the backlash if you did not install them.