The conditions needed for the ECM to run the EGR system test are such that it rarely happens. The factory manual details it in the troubleshooting steps for code 32. Look in the Engines section here on the forum and in the sticky'd posts near the top you'll see a link to online PDF's of the factory manuals; very handy to have.
If you've "uncorked" your exhaust (anything other than stock manifolds, catalytic converter, and muffler) then it's possible your unheated (1-wire) O2 sensor isn't getting hot enough to generate the expected signal. Converting to a heated (3-wire) sensor may help. When the test conditions are met, the ECM will momentarily operate the EGR valve then watch the O2 sensor readings to "see" if it had any effect, so you see why the O2 comes into play here.
It's also possible you replaced a functioning EGR valve unnecessarily, and the aftermarket valves are often not directly compatible regardless of what their application notes claim.
Trust me go read the manual on the code 32 troubleshooting. It'll shine light bigtime on why you're randomly getting the code. If the truck doesn't run any different, especially if you're not noticing pinging under part throttle or load, probably nothing to worry about. Again, consider the 3-wire O2 conversion. It's been discussed a bit here recently - those posts will be in the Engine section, too.
Richard