Car wouldn't move, got it towed to dealership and now it's running fine

Just throwing out ideas here. In most cases the engine runs, so one would suspect something in the clutch-transmission-drive line areas. In other words "after" the engine. It is certain something was wrong, and if the dealer can't find it yet. Ask them to try again. tell them its a safety issue for you (they may need to keep it over night to see if they can replicate the trouble). Regardless they should give you more information than " there's nothing wrong with it". Ask to speak to the service supervisor and get his/her input and what they suggest you do to recreate the problem or what to do if it comes back. To be fair it is nearly impossible to correct something that they cant find. so any information as to what you were doing: time of day, your speed, road conditions (smooth, rough, paved, dirt?), weather (dry, wet , snowy ?), time of day, Truck warmed up?, still cold from just starting out?, Driving straight , just making a turn? parking brake on/off? or anything else you can think of. Good Luck