I haven't noticed it with our current blazer, but our previous one did. It had the exhaust routed out the sides behind the back tires as well, and it didn't help.
Just have to seal the tailgate area as mentioned above.
Our current blazer has the factory small rubber strips around the tailgate, but I found that really wasn't enough, and after years of use they compress to the point that they don't conform anymore. It's such a skinny strip anyway that I don't think it serves much purpose
I went accross the bottom of our tailgate with sticky sided foam just below the factory rubber strip as an added precaution.
It must have done a better job of sealing because my wife and I noticed right away the truck was much quieter inside, without the exhuast rumble and road noise coming through the back.
If It's sealing out the noise then it must be keeping out the fumes as well. Doesn't put much faith in the factory skinny rubber strips (for me anyway) I'd like to see those rubber strips stick out about an inch or so,,,,and tailgates just aren't the tightest hinge design anyway, and have some play in them.
Just a thought.