Sorry for the late reply, I've been away: MDindiana said it well: benadryl is NOT the drug of choice for anaphylaxis: injectable epinephrine is the mainstay, such as an Epipen - Which is expensive. Individual vials of epi. and syringes are cheaper, but harder to come by. A willing physician may be willing to prescribe either for someone without a clear personal diagosis of anaphylaxis/severe allergic reactions.
Having said that I've never had opportunity to need it in 40 years of office practice, nor outside the office; so I haven't kept it in my car kit. "Expiration" dates on it are probably "real" as well, being a liquid: I think about 2 years after purchase. So spending perhaps a over a hundred dollars every two years to replace an epipen is a factor to consider.
If the need exists however, and all you have is an expired epipen, it would be better to use it than do nothing IMHO.