the planet gears do whine a little IMO when in gear at slow speed

at least all mine have had what i think is the tranz planet gears whining

rear chunks can tranzmit noise up the shaft

but..rear axle most of the time drowns it out when bad from my experiance

dont think you will hear it in N or with the drive shaft out if its the tranz gears

swaping to the "used" 3.91 is not the docs work IMHO

just cause the gears and diff looked good is another thing

"used" is key word here when it comes to the rear axle makeing noise