The EGR valve introduces oxygen-poor exhaust gasses into the intake mixture
to lower combustion temps, thereby lowering NOx emissions.  The EGR valve is
only supposed to operate at cruising speeds.  If the EGR is fubar and
opening when it shouldn't (or even stuck open, remaining open all the time)
it will cause driveability issues.  If the EGR valve is opening too far at
cruising speed (like you describe in your post), it will also cause the same
problems.

Yes, if the EGR is malfunctioning it can cause misfires and a lean mixture.
The O2 sensors detect less oxygen in the exhaust flow, and the PCM eases up
on the fuel delivery, resulting in a lean running condition.

You have a linear EGR valve on your truck, which is going to be expensive.
You can remove it and check to see if the pintle is stuck open with a chunk
of carbon.  From the sounds of it, it's opening too far and likely needs to
be replaced.  If it was stuck open you'd have a really bad idle, stalling,
and miss at all speeds.

Doc