I've looked at a few threads and haven't found something close to the problem I was having. I was filling my 98 Blazer up with gas and took the cap off and a lot of air pressure released.
I thought it was strange but didn't think that much of it. About an hour later while driving I could hear a strange humming coming from the rear end. I stopped and got out and looked around and could hear the noise coming from the gas cap.

I twisted the gas cap off and air came rushing out. Not like a little air like a huge amount of pressure that lasted like 15 seconds then the engine died. I put the cap back on and started the Blazer and it started fine. I opened the gas cap and air just continually flowed out again for 10 seconds or so and the motor died again. It started right back up again and ran fine until I drove a few more miles and shut it off. The next morning it started fine and no pressure on the fuel cap and no codes at all during the whole thing.

A couple weeks ago when it was 100 plus degrees outside I started it up it ran really rough and died and I had to start it back up and it ran fine once I took off. No other problems until this happened. I've looked at a lot of the EVAP problems and troubleshooting but I haven't really seen the extreme air pressure coming out of the fuel tank and no codes. I've also been driving this on rough forest service roads recently and its very dusty.