You need to remove gear box. It has been a while but its not too bad, biggest thing is its HEAVY we tied a rope to it and around something above it and lowered it. Getting it back in is a bit more difficult similar way but harder to put something back then out.
I used a bit of a gin pole on the front end loader to deal with it. Of course I was a one man operation. I had plenty of trouble getting the box apart and back together. Things were tight, too much hammering,but it lasted.
There should be an access panel in the grain tank, take it off and run some bars across the tank and fix a block and Tackle to it to lower and raise box. I have rebuilt one and I agree it was a bit of hammering to get it apart. Probably easy for someone who knows what they are doing and have some special tools. But it did work and I had no trouble with it. Grease that coupling everyday and you will never wear it out.
I have bolted a small chain to the top of the box and connected it to a 2"strap that wrapped over the return to cylinder auger and then used a skidsteer to let it down it to the ground. Pay attention for any shims on the gearbox when you take it loose. It may be possible that your gearbox needs to be aligned if you are chewing up the splines.