2001 4.0L, I ve been tryin to figure out whats causing a deep bass vibrations underneath the driver side seat at 25-35mph (its not taco bell). I just put in 4 in spring in the front, a 3.5 in add leaf, and 1.5in shackle, I feel like it could be a u joint or the transmission but I don't know. every time I almost let all the way off the accelerator, I get a rumbling that sounds like a tires rubbing, and when I barely push back down on the pedal the rumbling returns. Its not the tires, i took off the larger one and put the originals back on with the same outcome. Any help is very appreciated

 

get a tcase drop

 

It's probably some drive line vibes from doing the lift. You increased the angle between your transfer case and your rear axle. You can get vibes from doing a lift. I have a 3" lift on my XJ and had some vibes, just as you're describing. I did a transfer case drop. It's cheap and easy and got rid of my vibes. Here's what you need.

http://www.quadratec.com/products/76006_801.htm

It took me about 20 minutes to install.

 

Yup. That's what it is.

Tcase drop, SYE, or bring the lift down to 2".