Seems to be either in the rear end or maybe tranmission… how do you tell for sure ?? (5 speed manual trans )
At 180,000 miles which is more likely ?
I assume the whine appears as you accelerate and the tone rises as you go faster. To differentiate between the transmission and rear end, shift into your direct gear which will probably be 4th gear. If the whine is still present as you accelerate, the rear end is suspect. If not, the transmission is suspect. Also the volume of the whine will sometimes change with the gear selected in a transmission i.e. louder in first or maybe third.
I also assume that you have checked the oil level of the transmission and the rear end. It is probably too late to gain anything by changing both fluids now. But, you might get a few more years out of the truck if you do change the fluids. You probably will not cure the noise but getting some of the trash out of the units might help.
Hope this helps.
with ford rangers the whining noise usually comes from the owner when he or she is under the carriage looking for the latest broken part.
Shift it into neutral and see if it continues.