My 931B suddenly barely moves. Everything was fine until I drove 4 miles down an icy mountain road, 25° temp, top speed (3rd gear). Drove a hill with left track in snow for traction. 1, 2 & R, moves a bit, nothing in 3d. All fluids & filters full & clean

Check the transmission pump suction strainer.

Let it warm up. It could be ice in the strainer.

Fixed it! Control rods for brakes - external, under seat - were seized. Thanks for suggestions.

Not necessary to replace the strainer unless it is damaged. Just clean and replace.
If that doesn't do it you will need to get pressure tests on the transmission. Looking for leaks or pump problems.

check the the simple things first. sometimes it is something so simple you wouldn't think of it.

a nearby mine had a haul truck that wouldn't start they tested everything and the service tech believed it was a bad engine control module 14 grand later and it still wouldnt start so they refitted the original. the apprentice decided to check the fluid levels it was low on oil topped it off and hey presto it started with the original module.
the watch dog was interrupting the start protecting the engine from lack of oil.

and no the dealer wouldn't accept a return on the part. so they now have 14 grand worth of paper weight sitting on the shelf.