When driving in 2WD I hear a rattling/grinding sound?

This sounds like you need to check the rear U joints for wear, or you have a transfer case that needs to be rebuilt or is low on fluid. If you upload a short video of the problem, I could tell for sure what's going on. Here is how you can remove the drive shaft to check the U joints:

https://www.2carpros.com/articles/how-to-remove-a-drive-shaft

Check out the images (below). Please let us know what happens.