How many miles are on your tired truck? Have you checked your Under Valve Cover Harness on each side to ensure the plug is fully inserted? The clips get weak and wiggle loose. If you are going to do this, then this modification is called the 50 cent mod on this forum. Do what these guys above have recommended for looking at the thermostat and check your fan clutch to ensure operation. The drivers side valve cover will take just less than an hour to remove to inspect the harness. The passenger side take about an hour to remove. I can post a video from youtube for the 50 cent mod if you are going to do it.
Don't take your vehicle to the stealership
This forum is really good at giving you the info. You just need to apply the info in the form of wrench in hand and eyes on truck.
Did I mention that I hate me some stealership.
Dwayne