Goood morning and welcome to JustAnswer!. This is Ed.
Oooof. Loud single-click starting problems like you've described almost always wind up being a bad starter, especially with the 3.3/ 3.8 Caravan engine. Being intermittent, the starter might have been having a good day when it went in for testing and the bumping and bounding it got on the way to the store probably improved its chances of putting on a good face. I've hit the key 100 times consecutively on suspect starters without fail, only to have it act up later when not expecting it. They do this kind of thing.
To further narrow it down, you should monitor your system voltage during one of those single-click no-start events, as you're still holding the key in the start position. You should see voltage as high as before you turned the key (well... a couple tenths lower, maybe). If voltage stays high and the starter still won't engage, turn the headlamps on while the key is still turned. If you had lost a body ground, the headlamps would be dim or worse -- bright lights suggest it's OK.
Testing the engine block for loss of ground would be next. Set your meter up on the 20V DC scale, then connect between batter negative and the block. These are both negative sources, so as long as their voltage potential doesn't change, your meter will show 0.00 volts. Now crank the engine while watching the meter. Some voltage drop is normal in a DC system like this and I expect that you will see up to almost one full volt of drop during actual starter operation. But if you see zero drop when the starter clicks but does not crank, it shows your ground is good. A large difference in potential would show something very near 12 volts, as the engine has lost its ground and is now electrified with positive potential. Fix the bad ground and try again if that happens.
All that remains is the positive cable to the starter itself. If you can connect to the starter at the point where the cable connects and get it to act up, voltage should be the same there as it was at the battery. At that point all that remains is a bad starter. One that made you take it out for testing before it gets replaced. I hate when that happens.