I can attack this two different ways but I will begin with the W-body rather than the G.
These fail every 6 or 7 years it has nothing to do with mileage for some odd reason. I have replaced them with 20k mile and 250k miles and it seems to be a duration thing rather than distance. They will throw a missfire code or may throw a crank sensor code or both. Wells (Hecho en Mexico) unit availible from Autozone for $58, new GM also (Hecho en Mexico) costs $218.
1. Remove negative battery cable before perfomring ANY electrcial work, then jack vehicle up and support car with jackstands outboard on left and right sides under corners of rear of subframe.
2. Remove plastic inspection cover from starter and transmission (2) 10 mm bolts securing it on, pull down to remove.
3. Remove (2) 15 mm bolts securing starter to front of engine block, you will need to pull starter snout out of recess cut into transaxle towards passenger side of vehicle and manipulate it down between front of subframe, becareful not to drop this unit as its heavy and the ignition wires are no long enough to set starter on ground with out removal.
4. Remove 7 mm bolt securing solenoid wire onto starter/solenoid assembly.
5. Remove 13 mm battery wire from center of solenoid.
6. Set starter/solenoid aside.
7. Crank sensor is located directly above starter unit on front of engine see diagram.
8. Remove crank sensor pigtail it has 4 wires andis a flat harness.
9. Remove 10 mm bolt securing crank sensor.
10. Turn Crank sensor 90 degrees and pull out.
11. Reinstall new sensor plug sensor in, reinstall igntion wires , reinstall starter/solenoid assembly reinstall inspection cover, bring vehicle to floor level.
Many times you can clean the metal shavings off the magnetic plug type pick up and clean oil and grease form it and restore the signal but for my time/money I am buying a new one, or taking one of my 3 that I have purchased from Autrozone back for a free replacement. This job should take 15-20 minutes.