If you were to unbolt the drive shaft at the rear differential, you will see how the yoke (flange that drive shaft couples to) is mounted: there is a 30mm (in my case) lock nut that holds the yoke on to the pinion. Tightening this requires a small inch pound torque wrench. You tighten the nut and then check the drag (bearing pre-load) by turning the shaft with the torque wrench ( dial type is best). For my truck, the resistance should be about 6 inch/pounds (NOT FOOT POUNDS). The only way to do this accurately requires pulling the axles out so that there is nothing affecting the movement as you do this.
If your truck has been re-geared, chances are the gear ratio is different than stock. A quick and dirty way to check this is use a gps and track your speed on the highway, if there is a significant difference between the speed on your speedometer and the gps, chances are the gears are aftermarket. If the speeds are the same, double check to make sure there isnít some kind of speed calibrator wired into your tachometer. For Toyotas check the transmission, thatís where the speed is read and thatís where the calibrators get wired in. You will have to factor in tire size too if they are not stock size.
As for the noise, mine was a hum in relation to the vibration that was being caused by the pinion not being pre-loaded into its bearing by that nut. I donít know what to tell you about a squeak. That could be a multitude of things. You need more information to properly trouble shoot. Does the squeak get loader? If so, does it get loader during an increase of speed or rpm? What are the road/ environmental conditions when it happens (i.e. all the time, in the rain, bumpy roads)? Change the oil in suspect components, be sure to drain oil through cheese cloth or some kind of strainer. What kind of particulate does this produce (there will always be some fine metal particulate due to normal wear)? If you isolated it to certain components try and move them by hand and see what you feel. For example, when I had my axles out and drive shaft disconnected I could easily turn my differential by hand, if had felt even the slightest amount of graininess in my bearings (be sure that is what you are feeling, I am a journeyman machinist so I know whatís what as far as this goes) as I was rotating it, I would know there were bigger problems that would require at least a bearing replacement.
Sorry I couldnít help directly with the problem, but you canít go wrong with some solid trouble shooting and critical thinking. If you donít have them, a couple hundred bucks for the Toyota shop manual for your truck is worth every penny.