I have a 2003 Suburban 5.3L (vin Z) RWD with ~120K original miles. It's a bit of unicorn we got with 30K original miles in 2016. I tend to keep up on all maintenance b/c I love this thing and want it to last as long as possible.

BUT, it has had an intermittent issue the last ~2 years. Occasionally it won't start at all. There's lights on the dash. I try to turn the key to run, but the starter doesn't even engage. If you move the shift lever all the way to the left, while in park, then barely move it to the right (find the "sweet spot"), then hit the key it fires right off with NO issues otherwise. It drives completely normal at that point. I have replaced the neutral safety/range switch on the transmission, the ignition cylinder, and a related relay. So far, none of those have made it go away. It doesn't happen with every start. In fact, it's probably been at least 6-12 months since it last occurred and has now happened 4 times in the last week or so.

It has NEVER thrown a related code as far as I know. I had a code for a downstream O2 sensor 6+ months ago and replaced the sensor. In the last 2-3 weeks it threw a "too lean" code for both sides. Turned out the intake manifold gasket was leaking, so I had that replaced. While it was in the shop, I went ahead and had them change the fuel filter too (I figured it was a bit overdue and didn't want to wait until I had time to do it myself). He also did an oil change and coolant flush as part of the gasket replacement.

I'm guessing it's probably in the steering column somewhere given that it's entirely based on the shift lever position. Maybe some plastic piece or bushing is wearing out, so it can't always detect that it's in park?

Any ideas what else it could be or what I should check next?

Edit: I just checked both the PCM fuses and they seem fine.