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 employed by a farmer, you can operate under the farm exemption.  However, if you are employed by a custom combiner, a CDL is required. A restricted CDL can be obtained based on custom combine work.   

There is no knowledge test or skills test for a restricted CDL, but there are many other limitations, view CFR 49.383.3 (F) for an outline of the limitations.