First.. thanks to all of you who replied, it does help a lot to hear the opinion of people who have had some more experience with their trucks than me.
This is the result of my tests:
Battery and engine both seem fine.
(Battery had enough charge, I was able to turn motor over manually all the way with the big flywheel on the car's belly.)
So, on to the starter:
I removed the starter motor from the car to do some tests on it:
the solenoid came on, moving the gear into the window, but the motor did not rotate.
I looked into the contact assembly of the switch and found a heavily worn contact for the motor's power.
I was able to fix that by turning the contact 180 degrees so that it exposed a non-worn side to the switch:
After reassembling, the motor seemed to work fine... yay!
But this is the situation now:
I put the motor back in half way, so that it doesn't actually crank anything, it just runs being electrically connected to all its proper cables with an added handmade negative contact:
Everything worked (both solenoid and motor)
When I put it back in all the way however (so that the starter gear will get in contact with the engine flywheel etc.), the solenoid continued to work, but the starter motor would not turn the engine at all but appear to get stuck instead.
The solenoid produces the "thud" sound I am talking about in my original post, but the motor doesn't turn.
I verified this by listening, unfortunately I couldn't watch what exactly happens when I turn the ignition key.
It definitely does not crank. There is a very silent winy sound after the solenoid click.
So, it seems that something is still wrong with my starter motor itself.
So here are my current questions:
In my repair manual it said that there are some parts that come loose when the starter motor is removed - some things that needed to be put back in sequence when it is reassembled.
Well, I didn't get anything like that:
All I had to do was loosen the 2 bolts that hold the starter to the chassis and remove the 2 nuts that hold the electrical wiring on the contacts (btw, which are a total pain to both remove and reattach, completely unreachable..), then it just dropped down/came out.
Could my starter be missing pieces, or is it just too "weak" to turn the engine despite running fine when I test it separately?
My next step is to take the starter and go to an auto parts store for testing.
I'm sure they'll love to sell me a new one
Thanks again for helping me! It's somewhat fun to climb around down there fixing it, but I hope it'll all lead up to a happy end...