Shaking front end in low gear, 2013 HDPTO

Hi. I've got the 2013 HDPTO with about 450 hours on it. It's been a great machine, no previous issues.

But just took it out for 2nd time today and noticed a very substantial shake in the front end when in low gear drive - and the shake seemed to go away at higher speed. I always replace the front drive fluid every 50 hours, and just did it about 5 hours ago, but I was still worried the front drive was the culprit so I drained the fluid out to see if there were any metal chunks or anything, and it was perfect.

When I took it out earlier today I did go through one wet and muddy spot - so maybe some mud is causing an issue? It wasn't very deep though...

Anyone experience anything similar or have an idea? I hate to send it to the dealer if it's something simple.

Thanks.

Dan