Worsening rumble during 2WD on highway

[MichaelAngelo]

This is a worsening rumble-strip-like vibration that starts at about 80km/hr on the highway and worsens if I try and go faster. It disappears if I lift completely off of the gas pedal and returns when I try accelerating again. Over time, it's been getting more and more severe.

Two different people have looked at the U joints and said they seem fine. One shop lifted the rear end up and discovered a "clunking" when manually moving the rear wheels between forwards and backwards. I can replicate this clunking by shifting into forward, moving, then shifting into reverse and moving again and vice versa.

378,000 km this truck has served me well. On a road trip this summer I blew a tire and replaced the two rear tires with slightly narrower and higher ones than the front wheels. The technician that discovered the clunking suggested this is what caused this problem.

I discovered that this rumble goes away if I switch to 4WD... so for the past few weeks I've been burning up my drivetrain driving it in 4WD on the highway. I want to have some idea of what could be going wrong and if the truck is a goner before I bring it in for a mechanical fishing expedition .

edit: one thing I should add is I recently discovered a "clanking" when driving over bumps. Maybe something is severely loose and rusted free down under... but it was up on a hoist not too long ago and nothing was mentioned to me...

 

[MichaelAngelo]

bump? Any ideas guys?

 

[Cree]

bump? Any ideas guys?

Drop the rear drive shaft and drive it in 4 high. See if the noise persists. If not, it's something in the rear differential. Rumble, my money would be on carrier bearing.

 

[MichaelAngelo]

Drop the rear drive shaft and drive it in 4 high. See if the noise persists. If not, it's something in the rear differential. Rumble, my money would be on carrier bearing.

Thanks! What do you mean by "drop the rear drive shaft"? When I do drive in 4 high the noise is gone.

Could the carrier bearing also cause the "clunking" sound when switching from rolling the truck forwards and backwards?

 

[gamefreakgc]

If you still have larger tires in the back do not use 4WD! You will blow up your transfer case.

#1 vote is still the driveshaft, you can have it looked at all your want but if it's out of balance the u-joints don't matter.

#2 vote is like others said, there's a problem with a bent axle or warped axle housing.

 

[MichaelAngelo]

If you still have larger tires in the back do not use 4WD! You will blow up your transfer case.

Ugh oh... it's been several thousand km at this point in 4wd on the highway...

edit: with a burning smell.

 

[gamefreakgc]

Ugh oh... it's been several thousand km at this point in 4wd on the highway...

edit: with a burning smell.

With larger tires in the back, the rear tires are making less revolutions than the front, which puts a lot of stress on the gears. These transfer cases can take a beating, but I suggest going back to 2WD otherwise bad things will happen!

 

[MichaelAngelo]

With larger tires in the back, the rear tires are making less revolutions than the front, which puts a lot of stress on the gears. These transfer cases can take a beating, but I suggest going back to 2WD otherwise bad things will happen!

Thanks for explaining that to me, I previously didn't understand how would cause any problems. The one guy who discovered the clunking sound when turning the rear wheels one way and then the other said that the taller rear tires were what caused this rumbling to begin with (even though I was in 2wd the entire time leading up to the problem). The rumbling is also only in 2wd.