The TX out pin is also CK in, so they have a PPS in register named TXPPS-makes sense, right? why name it CKPPS?
The register is in the header file, but its confusing when they name the output function as the input name, I've run into this on 16f (I think they also sometimes combine like TXCK, gets confusing).
Now, to come up with an output source selection, since the datasheet does not list TX/CK, there are 11 reserved of the 31 available to try (they list 00000 as reserved, but that has to be LATxx, so they have more than one datasheet problem). Maybe they have a similar 18f with a datasheet that works.
In the RxyPPS register, put the number of the peripheral output you want in there (0-31, 0 means use LAT). Fire up the TX spitting out something, try the 31 combos in RxyPPS, when you see something you hit the target.