Switching of RS485 differential line. Two sources, one output
As you don't wish to use a relay, you could possibly use a CD4066, CMOS Quad Bilateral Switch (datasheet):

 

Although as suggested in Finbarr's comment, as you might want to use a double pole switch, you would want to use all four switches within the IC package.

See image below taken from How to Build a 4066 Quad Bilateral Switch Circuit



Just modify the schematic to use the other two input and output pins as well as Control C and Control D