It's the gearbox, almost guaranteed. You should belong to the Open Range owners forum. Someone on there just changed theirs yesterday. 2016 is the peak year for failures, since they redesigned the gearbox in 2015 and that turned into a disaster. It took a couple years for BAL to get it right. The new replacements seem to be ok.
Dyersrv.com is usually the best place to buy. R25076. I have the part number memorized for some reason.....
JK
Edit: I've done 2 of them and if you can turn a wrench it's easy. Put the slide halfway out, support it outside. Remove the wood trim, and like mentioned above, mark the chains and cable adjustments with Whiteout and put everything back in exactly the same position when done.
Watching the YouTube videos helps a lot.