Can you drive a tractor trailer in Illinois for farm?

You need a CDL.

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If the vehicle is a registered farm vehicle, and is operated by the owner of that farm, their immediate family, or direct employees of that farm AND that vehicle's sole operation is for the purposes of that farm AND it is not used on a for-hire basis, AND is operated only within a 150 mile radius of that farm (by the federal regs - state regs may lessen the operating radius), then it is not considered a commercial vehicle under the FMCSR, and does not require a CDL to be operated. Licencing regulations for farm vehicles, RVs, etc. are left to individual states. While you wouldn't need a CDL (provided all the above conditions are met), you would have to upgrade your licence to whatever the state of Illinois requires.