Anyone might know what is wrong with my truck?

Whenever I drive my Ford Explorer 2002 4.6L 4x4, I slow down and a loud humming noise comes from under the truck, I think its the transfer case grinding on the drive shaft, or maybe low transmission fluid? Anyone might have any opinions on what this could be causing the strange humming/grinding noise? I cant afford taking this thing to a mechanic because they always rip me off in this area and they always charge for diagnosis. Thanks!