My 2003 blazer dies at stops and only runs up to 10 mph

It could be fuel starvation. You may consider getting the fuel system serviced, changing the filters and checking the injectors for pressure and free flow. That is where I'd start. Next would be electrical / computer input from sensors, the OX sensor, and plug it into an error reader to see if the computer itself can tell you the issues.

The 03 blazer has a 4.3 v6. They don't have fuel pressure sensors? Are sure you've selected the right vehicle? Also I think a clogged cat is a possibility, the 4.3 shouldnt run at all under about 45 psi of fuel pressure. To test if it's your cat, take an upstream o2 sensor out and go for a test drive. If you see an impovement then it's a clogged cat. The opening from the sensor not being there let's some exhaust flow.