At 20k isn't much of anything to do. As previously recommended check fluid levels. Do a sanity check stick your nose in and under the truck looking for any signs of previous damage or maintenance.
Possibly change the oil, if you don't know the last time it was changed.
Scheduled Maintenance (from the manual):
At every oil change interval as indicated by the information display
Change engine oil and filter.**
Rotate tires, inspect tire wear and measure tread depth.
Perform multi-point inspection (recommended).
Inspect automatic transmission fluid level (if equipped with dipstick). Consult dealer for requirements.
Inspect brake pads, shoes, rotors, drums, brake linings, hoses and parking brake.
Inspect engine cooling system strength and hoses.
Inspect exhaust system and heat shields.
Inspect front axle and U-joints. Lubricate if equipped with grease fittings (Four–wheel drive vehicles).
Inspect half-shaft boots (if equipped).
Inspect steering linkage, ball joints, suspension, tie-rod ends, driveshaft and U-joints. Lubricate if equipped with grease fittings.
Inspect wheels and related components for abnormal noise, wear, looseness or drag.
Every 30000 miles (48000 km)
Replace climate-controlled (heated and cooled) seat filter (if equipped).
Replace engine air filter.
At 100000 miles (160000 km)
Change engine coolant. Every 100000 miles (160000 km)
Replace spark plugs.
Inspect accessory drive belt(s).
Every 150000 miles (240000 km)
Change automatic transmission fluid and filter.
Change front axle fluid (Four-wheel drive vehicles).
Change rear axle fluid.
Change transfer case fluid (Four-wheel drive vehicles).
Replace accessory drive belt(s) if not replaced within the last 100000 miles (160000 km).