2009 Expedition XLT 5.4 liter engine with 208k miles. Former Highway Patrol. Purchased six months ago at 206k miles. Two short episodes (3-5 seconds) of severe shaking at rough idle and in reverse. Drives great on the streets and highway. No check engine light. Codes P0341 & P0346 -Camshaft position sensor. Very regular dealer oil changes. Smog shop tech says EGR and all emissions control system is working fine. 99%+ of the time idle is smooth. Is it just the camshaft position sensor(s), or more likely something bigger? VCT Solenoid(s)? Timing chain? How long will this truck keep rolling without a major intervention? Just keep driving it? Start throwing money into diagnostics or new timing chain etc? Or, walk away? Is it safe to drive? If it is timing chain stretch, how long will it run until it dies? What should we expect?