2000-2500 RPM vibration

[Sscolax]

I have a Bronco II with a 4.0 and M5OD out of a 91 Explorer. Lately I've been experiencing a vibration that's dependent on engine speed. It starts at about 2000 RPM and goes away at about 2500. It doesn't matter what gear I'm in, but it only happens when the drivetrain is under load. Accelerating is worse, but it's there while decelerating as well. With the clutch disengaged or the transmission in neutral there is no vibration. My guess is that it's a bad tranny input shaft bearing. Am I right? I'd like some ideas that aren't mine before I pull the tranny to get it taken care of.

 

[BrooklynBay]

Did you check the U joints? Are the tires properly balanced? Since you mentioned that it's dependent upon RPM, does this happen while you rev it in neutral too?

 

[Sscolax]

The speed of the vibration is actually similar to a bad U-joint, not like an unbalanced tire, but the U-joints are good. In neutral I can rev it from 0 to over 3000 RPM with no vibration, but it returns as soon as there's a load on the driveline. The peculiar thing is that it peaks at about 2200-2300 RPM in every gear. Going from a stop to 65 MPH I feel the vibration come and go 4 times as I accelerate through gears 1 - 4, and when I reach 65 in 5th the vibration's just becoming noticeable again (about 2050 RPM with 32" tires and 4.10 gears). When I've had bad U-joints the vibration was always dependent on the vehicle speed, not the engine speed. Same with unbalanced tires, except for the resulting vibration's much lower frequency. I also had a set of 33" tires on recently and the vibration I'm experiencing was identical then.

 

[Sscolax]

I never reported what this problem turned out to be. I had a broken exhaust hanger, and the catalytic converter was contacting the t-case skidplate when the driveline was under load. This caused the vibration to transfer into the frame. I think the exhaust is somewhat resonant at about 2200 RPM, so the vibration was the most intense at that range.

So if someone else experiences the same symptoms this is definitely something to check!