Labs, and pits are both very friendly dogs and terrible guard dogs, aside from their size, looks and a pit's reputation (though, I have more fear of a chihuahua than a pit). I would say, in your case, probably going to be better to get a better fence and landscaping to deter thieves. (High fence, thorny, thick hedges, etc.). I rescued a very badly abused dog (a catahoula) who is both large and very timid toward people. He's turned into a good guard dog but not by intention.
As a general rule service dogs are trained to be accepting of strangers, and social situations. You can try rewarding your dogs for barking when folks are around, try with friends or some such, starting with a "speak" command, and then transition into only using that command when people come around. Eventually they associate people with the command. You can also teach them certain bad habits, such as jumping and such. I doubt you would ever really be able to make them aggressive.