Why do my lugs on my back tire continue to back out when the tire is balanced w/the right lugs and new studs?

My rear drivers side tire came off my truck twice. I just bought it and it had 37" tires on it. Now it has 33's and all new studs and lugs that go with the aftermarket rims. it's a 2004 Dodge ram 1500. It is just the one tire now and there is no shimmy going down the highway. The lugs are torqued firm and it still occurs after driving down the highway 20 miles or so.