I searched, but could not find any info about my problem. Hopefully someone here has some idea's.

1994 bone stock 4x4 pickup w/31's and a 5 speed.
My truck "grumbles" vibrates at a low frequency between 15-25 MPH in any gear, and no other speed. The vibration seems to follow the speed of the truck "it is something rotating"
It only vibrates while under power or engine braking, not when coasting at that speed.
If I coast from 30 mph down to 15 mph in neutral, there is no vibration but if I push or pull on the shifter "like trying to force it into gear" without pushing in the clutch, the vibration returns.
When I pull the rear driveshaft and drive it in 4WD front only, it does not vibrate.
I replaced both U-joints on the rear driveshaft.
I replaced the entire rear third member.

Any Idea's?

 

I was gonna say the rear diff, but if you replaced the center section and the noise remains then it cant be that. Did the noise change at all when you replaced the rear diff?

The only items left are the transfer case and transfer case/transmission mounts. Take a look at the mounts, if they are broken the trans/xfer could be shifting and contacting the undercarriage. Try changing the fluid in the transfer case, drive it and if the noise doesnt go away or change, change the transmission fluid and drive it.

Can you feel the vibration in either shifter?

 

the "grumble" is exactly the same with either 3rd. It is not a noise, you cant really hear it, it is too low of a frequency, it is just a vibration that varies with rear wheel speed.

You don't feel the vibration through the shifter, you feel it in the whole truck. Feels like going over washboard roads at 70 miles per hour. Brrrrrrrrrrrrrrrrrrrrrrr... Its not horrible, 85% of passengers will notice the grumble and mention it without being told to look for it.

I have changed the diff's, tranny, and transfer case fluid with all new synthetic.

I don't think it is the tranny, since it does it regardless of what gear I am in, and only when the rear driveshaft is hooked up.

 

Output shaft bearing or seal on the transfer case maybe?

 

No leaks but I was thinking the same thing maybe.
Rear output shaft bearing I am assuming.

 

Found it. $33 re-time and balance driveshaft.
Should have tried that first.