Shaking or rumbling feeling

Hello. I have a 2nd gen 2006 tacoma double cab TRD sport 4x4 long bed with 250k miles. 2 months ago, it began to have some sort of shaking or rumbling issue. It occurs while accelerating to about 30 MPH. It will be like a rumble-like feeling. If anyone has been on a long distance highway, it feels like going over the "stay awake" bumps on the side of the road to keep you from straying off the road. The entire truck will not shake though. I don't feel it on the steering wheel. It goes away when i take my foot off the accelerator. I'll then accelerate again and will not occur. I've had the suspension replaced but it's still having the issue. Tires are firestone and are about a year old. I've taken it to two "tacoma expert" mechanics but they have not found the issue. They say everything is fine. I live in southern california (LA/ OC county line) there's no rust. I'm just writing to see if anyone has had the same issue. Thanks in advance.

 

check the carrier bearing on your driveshaft it should have a little give to it, but not loosely flop around. a bad carrier bearing can be felt in your seat and a little bit of the truck as a whole, but you won't feel it in the steering wheel. if youve never replaced it at 250k, it's likely worn out and needs replacing, regardless of whether it's the cause of the vibration or not.

just run through the process of elimination......
-out of balance tires are generally felt at all speeds, getting worse as you accelerate.
-bad wheel bearings can sometimes be felt, but often are first noticeable by the sound they make.
-worn out leaf speings can wrap badly when you accelerate, throwing off your pinion angle and causing a similar vibration as a bad carrier bearing, but if you recently had them replaced, that's an unlikely cause as well

 

When my center bearing went bad, it felt more like thumping. I suppose it can feel differently based on extent of wear. I wouldn't rule out a thudding from bad torque converter. Also CV shaft, shaft needle bearing, or motor/trans mount.