* Driveshaft(s): This is a very strong possibility. A bent, unbalanced, or worn driveshaft (front or rear, depending on the 4Runner's 4WD system and whether it's engaged) will vibrate increasingly as speed increases. The location under the shifter makes this a prime suspect. U-joints within the driveshaft are common wear points and a source of vibration.
* Wheel Bearings: A failing wheel bearing can cause vibrations, but usually they're more noticeable at lower speeds and often accompanied by humming or rumbling sounds. However, if severely worn, it *could* manifest as described, especially if the vibration is felt more strongly in the center console area.
* Differential (Front or Rear): A problem within a differential (worn bearings, damaged gears) can cause vibrations that worsen with speed. However, this would usually be felt more generally throughout the vehicle, not necessarily concentrated under the shifter.
* Transfer Case (if applicable): Problems within the transfer case (if the 4Runner is in 4WD or AWD) could create a vibration, though it's less likely to be isolated to the shifter area.
* Tires and Wheels: While less likely given the description, severely unbalanced tires or bent wheels could contribute. However, tire issues usually manifest at lower speeds and the vibration would likely be felt more generally.
Why RPM is not a major factor: Engine RPM directly relates to engine speed. Drivetrain components like driveshafts, wheel bearings, and differentials rotate at speeds proportional to vehicle speed (regardless of engine RPM in a given gear). This explains why the vibration isn't tied directly to RPM.
Troubleshooting Steps:
1. Visual Inspection: Carefully inspect the driveshafts for bends, damage, or loose components. Check for any obvious signs of wear on the U-joints.
2. Test Drive in 4WD and 2WD (if applicable): If the vibration disappears or changes significantly in 2WD, it points towards a front or rear driveshaft or differential issue.
3. Professional Inspection: This is crucial. A mechanic can perform a thorough inspection of the drivetrain components, including a driveshaft balancing test, checking wheel bearings, and inspecting the differentials and transfer case for play or damage.
It's strongly advised to have a qualified mechanic diagnose and repair the problem. Driving with a significant drivetrain vibration can lead to further damage and potentially dangerous situations.
