I've heard and seen thousands of feral hogs; never heard one make anything like a growling sound. I've heard Bigfoot in a lot of southeastern states that made sounds like you described plus a dozen or more different types of sounds.
Just so you know, there have been over 200 Bigfoot sighting reports sent to the BFRO by Illinois residents. Counties in southeastern Illinois, in or around the Shawnee NF that have generated such reports - with the number of reporte from that county - are:Saline (4), Hardin (3), Williamson (4), Johnson (1), and Pope (1). (None from Gallatin.)
What you heard may have been Bigfoot. The area is ideal habitat.
Regards.