First off, hello ladies and germs! First-timer here,

I bought a 2008 Rubicon because my son wanted me to be able to ride with him. I took it on the trail and we hit some mud and while climbing out of the third or fourth mudhole, something started making a grinding noise (sounds like slipping gears but louder). I had to put it in reverse to get out and then Low gear to keep going and get home. I'm fine in Low or Reverse, but if I put it in Drive, which is hard to find btw so I'm hoping its a linkage issue, it idles fine; but when I give it gas it will try to go then "pop" out of gear and start grinding. I've read that it could be low on oil or no oil pressure, or a linkage issue among a couple of other things. Any advice would be appreciated.
Side note: I am mechanically inclined, but I know nothing about ATVs. I printed out the manuals (owners and service) and I just need some direction on what to try first.

P.S. Should I switch to Low when hitting mudholes, then back to Drive when the trail is drier? I went through mudholes getting off the trail after this issue and had no problems in Low. I'm a big guy and I'm wondering if I'm putting too much on it in Drive while in the mud.

P.P.S. The bike has a highlifter lift and 27.5 mud tires. The prior owner had plenty of money, so I'm hoping everything was put on properly by qualified people.