replacing air filter

[TXSUT]

You should be checking your air filter every time your change your oil, but after 50k+ miles, I'd say you are very long overdue. How often you actually replace it will depend on driving conditions. You could always get a lifetime filter like K&N (clean and re-oil at regular intervals).

Since you obviously don't know the history of your vehicle, I'd also advise getting all fluids (tranny, diffs, transfer case, coolant) changed at this time.