I'm assuming you've done the air filter and cabin air filters already as routine maintenance. You mention "differential" so I'm assuming you're 2wd. If that's wrong, you need to add the front diff and transfer case to the list. Depending on the age of your truck and number of miles, it might be time for a radiator flush and refill. I'd definitely do it if it's over 100k. Go through the front end and look for grease fittings (may be on the ball joints) and hit those with some quality grease if you have them. Check out your brakes (including the e-brake inside the rear rotor "hats" and consider replacing the e-brake shoes as the OEM ones are prone to delamination and cause pretty catastrophic failures if they do.