I allowed my dad to use the truck and he used 4wd then it wouldnt disengage and the fuse blew. he didnt know that so he drove 100 miles towing in 4 hi on the freeway. now a noise has appeared. when accelerating you can hear it. let off the gas and coast it goes away. Already filled the differentials up and checked atf fluid.

 

Have you checked your u joins? If they are stiff or are going out it can translate that sound up the shaft into the cab. Also your pinion barring is another possibility. When was the last time you changed your diff fluid ?

 

U joints seemed good and diff fluid was as soon as it did the noise about a week ago

 

Did you fill the diff or replace the fluid? If your u joints are stiff at all (only way to check is to pull the drive shaft) it will create a hum that is extremely annoying. Other than that all I can think of is your pinion bearing is going out work the symptoms you gave.