Need help with run away labs?

I have two labs they are both 4 years old, they are brothers. I had one at my mom's and my husband (boyfriend at the time) had the other. When we got married we got mine and outside kennel like his brother's to stay in when we weren't home. He was used to a fenced in yard with electric fence around the top because he wouldn't quit climbing out. Now we live on 60 or so acres and don't have a fenced in area so we bought an invisible fence for them. At first they both did great Hunter which is my husband's dog persay got out a few times because he was used to getting to run around where ever he pleased. From time to time he will run through the fence and roam the neighborhood. He isn't gone for more than a couple of minutes before we go after him because we don't want him to get run over or in a fight with the neighbors dogs. When he sees us in the truck he comes a running and jumps in the back no problem. My dog Gunner never ran through the fence because he knew from the fence at my parents that he wasn't to go near it. Well now he's the one running off because he ran off with Hunter one time now he just runs right through the beep warning then the shock. I don't want anything to happen to him because he's my baby and I just had to put my other inside dog to sleep and couldn't bear to lose another one. He know's when he's been bad but I don't know what to do. We play with him he gets exercise everyday he's in his pen for no more than 7 to 8 hours and out as soon as we get home. When we are outside with him it's like watching a child's ever move to make sure they stay right there with us so they both don't run off? The neighbors have been shooting any dog that comes in there yard and they've already shot another lab of a friend of ours and I'm afraid they thought it was one of ours. Please help me I don't want one of them killed.