The thick and thin fully-splined washers are two different Ford parts, with the FOTZ and F57Z prefixes, respectively. The thin splined washers are part of a set that includes a plastic spacer and another thin, unsplined thrust washer.
In addition to my earlier picture that includes the stuff that originally came off my front axle (far left), you can see a picture of the three "thin-washer" pieces apparently sold together at these two NLA product listings:
https://www.amazon.com/Ford-F57Z-3B458-A-WASHER/dp/B000O0DDNW
https://www.fordpartsgiant.com/parts/ford-washer_f57z-3b458-a.html
I'm honestly not sure what the original difference in application was between the thick/FOTZ and thin/F57Z washers. My front axle donor vehicle and the D35 Ranger I found in a junkyard were both '93, but used different versions of the washers. My '94 service manual shows 1 part, not 3, on the exploded view, and refers to it as a "spacer". This is what Ford calls the FOTZ, while the F57Z is a "washer". I think this implies that thick/FOTZ was original equipment in '94, but I really can't be sure.
In any event, if the thin washers were subject to breaking, and only thick washers are available anymore, the difference at this point is entirely academic.
I wouldn't feel comfortable running without the washers because the D35 TTB inner axle shafts are, as far as I can tell, also NLA. It would suck to misalign the axle shaft, blow the u-joint, and potentially take out the ears on a shaft that can no longer be replaced.