Kingpin lock pin
Since I am a ford guy not sure on the chev spindles..on the fords the little top and bottom caps just hold a grease fitting and don't do squat about holding the kingpin in place..
There is a kingpin retaining pin that goes in the center of the axle which lines up with a slot milled in the kingpin..sounds like yours is missing..
Thsi is a special tapered "bolt" that is held in with a nut..look at the other side to see if this is true on yours..
One thing to check is to remove the spindle and see if the axle end is egged..check that before reinstallation..
Sorry about the oops but good you caught it..
I have tried most all of it and now do what is known to work..