In a previous life, I was responsible for a fleet of ~150 semitrailers (as well as other vehicles). Our trailers did not get a lot of miles, so I had many, many 8- to 12-year-old tires on the road. In one 2-day period of running 20 or so containers from our facility in FL to port of Savannah and back, I bought EVERY AVAILABLE tire in my size on the I-95 corridor in south Georgia / north Florida due to blowouts!
At that point, I began a preventive tire replacement program. Reliability did not significantly improve until I had no tires older than 6 years.
My point is, you're close to that 6-year mark, and a blowout that tears up a dually fender will cost you a lot more than a set of tires. Put the tires on Craigslist and make them someone else's problem. Oh, and as mentioned above, if they're ST-, not LT-rated tires, don't run them on your truck at all.
My $0. 02...
kevin