Ah - another victim of the infamouse Death Cool. Other than this terrible engineering mistake, GM motors are super rugged. But if this truck is new to you and its been leaking coolant into the crankcase for a while, you may have some serious issues. Bearings are typically your first major item to be degraded by coolant, but you didn't mention how you lead numbers look. Is lead low?
Basically when your dealer repairs the coolant leak, they need to pull the oil pan and check the main bearings with plastigage. If the mains are good, you can bet the rest of the bottom end is good. Get these jokers to fix your truck this week and get your sales person involved. Tell the sales mangaer that you want it fixed properly, bearings checked and all, or you want a different vehicle. That sales person isn't gonna want to lose commision, so they'll beat up the service dept to make you happy. Be ruthless if you have to! Good Luck!