It might be in the steering post. disconnect the skis from the steering rods and move them independently. If those are free then move the rods around and check for tightness. Keep working your way up checking everything. I would say that 75% of the time the steering post is the culprit. They almost never get attention and can rust over the summer.
I had a similar situation last winter where a sled got progressively harder to steer as the season went on. We checked everything a dozen times and it all looked good. We ended up taking every thing apart like I described and it ended up being the steering shaft. Even though it took grease through the Zerk it would not free up. Changed it out with a used one I had and everything was perfect.
There has been a few people on this site with the same problem. Good luck.
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