Vibration 45-60mph and weird noise in rear end!

so i have a 02 Tacoma Prerunner 2.7 Automatic, Stock suspension 208K miles, i bought it used and since i had it, it has a vibration between 45-60mph once over 60mph it smooths out. things i have done so far:

Tires balanced
U joints checked changed bad ones
changed Center support bearing and had drive shaft balanced

still have vibration

i jacked up the rear end and have them on jack stand put the truck in drive and accelerate to 45ish and feel the vibration, so its coming from the rear of the truck.

rotated tires to see if one of the tires was still not balanced. no change!

also when i have the truck in neutral and rotate the rear tires i feel this rumbling feel in the differential... is this normal gear meshing?

when i had the truck on jack stands i tried to isolate the vibration if it was just one side so i lowered each wheel to the ground and spun up the opposite side, each time the vibration continued and it made this weird sound... could this be caused buy the spider gears?... should it be silent?

any help would be greatly appreciated.

I'm now thinking the rear end is bad (third member)... or bad rear wheel bearings.

 

Yeah unfortunately i think you're onto it there with your last two sentences......youve done a good job of troubleshooting and isolating the problem! So its prolly the 3rd or a bearing. Well actually its prolly the 3rd, you woulda been abke to isolate one bearing or the other when you were checking things while one side or the other was jacked up!

 

so i have been looking on Car-parts.com and found a low milage 96k complete axle/rear end with my gear ratio 4.56.... cheaper then the labor to rebuild my third member... I'm thinking i should try it.. it thats not it i could always post my current rear end on CL and recoup some of the cost. i really don't want to keep throwing money at this truck tho..