With my discount at the Chevy dealership..my cost was 162.00 for an OEM starter. Time to change it anyways

It isnt any of these guesses .

The phrasing of VECHILE spelling was the reason for cahs to rebel

Catch22, you are correct. The relay or solenoid was on the drivers side fender that old 68 Dodge truck. Very easy to service.

So I got the starter off. The motor spins when I touch a jumper cable to the terminal. Is the solenoid replaceable or all one unit? Local stores want $274 for a rebuilt so I ordered a new one on Amazon for $156.
What did you find Catch?

You will need to replace the whole starter. The solenoid is inside the starter housing, I'm thinking. Your symptoms are the same as my 2018 Silverado @ 100K. Replaced starter and good to go. I ordered mine through Rock Auto. Less than half the price of any local auto parts stores.

Probably a Ford. Dodges didn't have a relay in '68.

Had a 2000 Silverado that would do the same thing and the security would flash up on the display wait 10 to 15 minutes then it would start . I never figured out the problem just didn’t cut it off if I was in a hurry . The day I took it to the dealership to trade it we left it running over a hour in the parking lot .

Here's a good question for you to think about. You said the key doesn't work all the time. How about the key fob???? does it start every time with the push button???? Cause if it does that will eliminate about 95 percent of the other answers. I don't know the last time I used the key to start my silverado I always start it with the button on the fob.

My vote is for the ignition switch/neutral safety switch.

Our Monte used to do it, there's a wire in the column that goes to the security that bends with the wheel and starts to break to where there's no connection our mechanic done away with our security, the light stays on so a their would think the security is still intact, anyhow we haven't had an issue since.

Do the easy stuff first. Check all grounding straps. GM uses woven ground straps and they have a tendency to rot through. Some are visible from underneath the vehicle. Most of vehicle problems can be traced to a low voltage issue not triggering the correct path for voltage to flow.

Here's a link for example.
https://www.youtube.com/watch?v=8CFZcO4NDhI

I always go about it this way....

Test light to solenoid wire checks to see if ignition switch is giving it current. If yes, that eliminates all other options like ignition switch, neutral safety switch or relays if it has any and unless you have a bad connection at your battery, you are pretty dang sure its the starter.

One little test tells you where to lok.