I work for a custom combiner, do I have to have a CDL or can I drive under my farm exemption since I am hauling farm products?

If you are employed by a farmer you can operate under the farm exemption.  However, if you are employed by a custom combiner you have to have a CDL.  There is a restricted CDL that you can obtain based on custom combine work.   While there are no knowledge or skills test for a restricted CDL, there are many other limitations.  49 CFR 383.3(f) outlines the limitations.