I guess they figure if one is worn out both of them are.  If I am reading the parts book correctly the two pieces labelled 31 are a kit and available only as a kit (AR75769) with a substitution of RE47611 I don't understand the footnote reference of 024115

I did a search and found a used one at Wengers (http://www.wengers.com/parts/Farm-Tractor-Parts/John-Deere/4450/4450/Steering/4-WHEEL-DRIVE/PISTON-AND-SPINDLE-SET/01-AR75769.htm) for $ 373.00  Of course ... you have a used one.