u/Call-Me-Simon

My 2003 F-150 has a low hum coming from the back of the truck. The hum sounds almost like a low “rolling” hum, that gets louder and more high pitched as I speed up. It’s loudest at 50mph-60mph and the sound kinda disappears over 60 mph. I’ve confirmed that it is not related to RPM so I think it is likely related to my drive train. I have not noticed any changes in sound based on whether I’m turning. There is also a slight leak of differential fluid. I’m looking for advice as what to do next. I fear that I’ll change the differential fluid and the problem will be something with the gears and the problem will persist. I only spent $2k on the truck and she’s lasted for 4 years now. I will likely be buying another hand me down car in the next year so I’d rather not drop tons of money on this repair. Does anyone know what this sound could be whether it’s pertinent I address it?

Thank you for any help :)