Several years ago I had a similar problem, drove into a parking lot, went to leave and no starter, I couldn't even roll/jump start it. The problem was the power lead from the battery to the starter had come loose(not fallen off just loose). Check the main power lead and all of the ground wires to make sure none of them have come loose.

To check the starter try tapping the starter housing, if the truck turns over your starter is starting to go bad.

If the alternator was working well before, there isn't any real reason to replace it now.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed