I put this post together quickly so please double check with Messicks or other Kubota dealers before making any purchases.
Others have found U joints from NAPA
My thinking is that at the grass cutting time of year you need your machine back in operation quickly. Hence the logic to buy the complete assembled shaft. During the winter you can consider building up a spare shaft using various parts sources..
You state this is your first tractor and expensive U joints can easily be damaged when removing then with the intention of re-installing them. The tiny roller bearing inside tend to tip over during assembly and can cause lots of grief.
Kubota: Kubota RCK54-22BX(BX1800 BX2200) (54" SIDE DISCHARGE MOWER) Parts Diagrams
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If you study the parts list I think you will come to the conclusion that buying the complete shaft and keeping your original one for spares makes the most sense.
The complete shaft is #10 is $388.71
#20 the quick yoke which is all you absolutely need is $276.57
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#60 a simple U joint is $106
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Please do not hesitate to ask questions.
Dave