I think I found the solution.
I had only pull off the grease caps from curbside wheels. Both of these had those odd castle nuts. They are 6-sided, but with 10 slots. Very strange. Having more slots than sides, the slots have to be very narrow, which does not allow a straight path for the cotter pin, even thought he hole in the spindle is straight.
But today I pulled off the caps from the 2 roadside wheels. Sure enough, they the more typical 6-sided castle nut with only 6 slots (1 slot per falt). This allows the slots to be much larger, providing a straight path for the cotter pin. I tried removing 1 pin, and it came out and back in quite smoothly. So tomorrow, I will be picking up 2 new castle nuts with only 6 slots.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller