Really hoping you weren't running a lube pressure problem earlier, having a gear seize on a shaft will have the same effect as being locked in two gears at once, when in reverse some of those gears which idle on the shafts at times are turning backwards and if scored internally might have piled up...forgive me for posing possible worst case scenario, but I have seen similar in Deere syncro trannies before, and that is exactly what the gear box portion of a quad really is.
Your clunk has me thinking you got bigger fish to fry here.
As farmshop said, check your cables and even disconnect them and check the actual action of the shifters manually. Be careful doing such with it running, I would do so on a unit with issues like this with it well blocked up or on jackstands. And don't forget the front end as well. You can pull off the top cover as well if you can't find anything simple externally, and have a look at what happens when you try rolling it over by hand.
In my experience, nine times out of ten, shift cable issues originate in the console from things coming loose over time or improperly tightened or the cable hanging up inside the outer sheath. Check that they end of the sheath has not pulled away from the crimped on ends and that the rod end of the inner cable is not buckled or bending.
Contracts are made to be broken, a handshake is the law of God.