My Lovely 1986 auto Toyota Pickup: Bad starter or another demon?

So we have had problems with the starter (and flywheel?) since we acquired the truck, these problems being that when we would go to start the car, it would make a horrible “grinding” noise a few times, but would eventually start up. Recently, the car would not start at all, and made no noise when the key was turned. Called AAA, they gave us a jump, and the truck started again. Assuming the problem was then with the battery, we replaced it as well as both terminals. The truck would still not start after doing this, no noise at all when key turned. My questions then would be, if the battery is not the problem, why did the truck start after we had a jump? I’m thinking it might be the starter, and if the starter was shot, would the truck have no sign of life when the ignition was turned on? Should i try and change the starter or is there possibly another variable?

I’d clean the contacts and make sure that the cable makes good contacts. Follow where it goes to the starter and make sure that end is clean as well (be careful cleaning it - disconnect the battery first).
Also check the ground straps from the battery to the frame and from the frame to the engine.

As far as that grinding is concerned, it could be your starter’s solenoid. It may not throw the gear out far enough so it isn’t meshed properly. Worst case, it could be your flywheel but let’s hope that isn’t the case.