Thanks for the replies and welcome! Wish I had known this site existed ages ago.
We use the bike mostly as a farm vehicle (to feed, spray round up, just get from one point to another, etc.). I was on the farm next door riding home when it started. Was moving along in 4th gear at a pretty good clip down a long flat gravel road. Never mudbog it. It was running smooth and fine and with the throttle open it was almost like you had thrown the kill switch. It began to backfire repeatedly through the carb and sounded like it choked to death. Never saw any smoke of any kind. It jerked a little every time it backfired. It died, and I immediately cranked it back. It idled and would barely limp forward in first gear at a crawling pace. When you let off the throttle, it would and still idles perfectly as it always has. Oil in the airbox was about a small pill bottle full. Oil was warm, but not hot. Very thin, could have had some gas mixed with it. Breather and air cleaner were saturated in oil. When I removed the air cleaner, the back side of interior of the carb (airbox side of the main needle?) was black with a film of what looked like oil. There are no mods to the bike and factory all the way around without ever any issue. Only routine maintenance ever done, but fair to say it was overdue for an oil change and plug change (which have been done now).
I also have a 2004 model of the exact same bike. I brought the other bike over and changed plugs and wire (with coil) thinking the plug was fouled. Nothing changed. I have since put in a new plug. The bike has never been apart, but reluctantly took out the carb, took it apart and cleaned all jets thoroughly with carb cleaner, a small wire and blew it out with air pressure. It idles fine now, and doesnt appear to be putting any new oil in the box while idling. When you ease into the throttle it starts getting choppy then backfiring and will die if you dont let off. If you do let off, it idles just fine.
Hope this helps! Thanks guys!