vibration/harmonic sound hum???

Hey, New to forum..My 2000 Prerunner TRD 2WD 125K has a vibration and a harmonic sound hum at 50 mph. Does not go away at higher speeds. Feel vibration is seat and gas pedal as well as steering wheel. Her is where I am as of now.
Have: New tires (correct size) balanced tires, driveshaft, new u-joints, new rear bearing and seals. Noise and vibration are driving me nuts . Any thoughts?
Love the forum!!!!!Have gathered alot from just reading. Thanks.

 

Not sure if this is a shot in the dark, but have you checked or replaced your differential oil level lately?

 

Yes. Changed it when I bought the truck last year. Heard from the driveshaft dealer (driveshaft exchange)today and it did need balanced a little at no charge since he did it the first time when I put in the u-joints. Going to get the shaft in a bit. Will post if the problem is gone. If not I guess the next step will be checking the hubs to see if they are bad. I'll let ya'll know. Thanks so much!

 

Got it back from driveshaft exchange and it helped with the vibration, but still have a roar/hum noise. Probably going to wait for something to fall out and then I will know what the problem is. Thanks