I’m going to start trucking for someone else in their truck as a 1099 employee. Do I need an LLC or sole proprietorship?

For tax purposes, it does not matter.  Both are treated the same way when you are a single member LLC.  Income from both the SMLLC and the sole proprietorship will be reported on Schedule C.

 For liability purposes, the LLC provides protection that the sole proprietorship does not. 

What You Need to Know About Truck Driver Tax Deductions