Welcome to the garage. I would start doing a transmission service with new filter and fluid. You can have a look in the pan if there is any foreign material or metal on the magnet, which could indicate some internal problems.
I would also recommend servicing the differentials and T-case and do a full engine tune-up with air/oil filters, oil and plugs. Lateron you could tackle the coolant and brake fluid. (if the coolant looks brownish from rust you better change it as soon as possible. Brake fluid should be changed once per 2 years, because it is hygroscopic, it attracts moisture from the air. This moisture accumulates in bigger bubbles and can/will start boiling at high operating temperatures, causing air bubbles in the fluid and a spongy brake pedal or even total brake failure.)