When driving at 75 mph for extended miles, white smoke starts coming out on 2003 Nissan Xterra

For instance,it only happens when traveling at high speeds like on the interstate for extended periods. Any idea what causes it?

Agree with gg, check ALL fluid levels! Smoke may be caused by any of the vehicle fluids being blown onto hot exhaust system at that speed.
I suspect trans. fluid leak.
1 more answer
if the white smoke is coming out of tailpipe you might have a head gasket problem,check anti freeze level