I plan to install a starter cutoff switch on my 2000 4x4 Blazer to make it less stealable. My question is, can I install another cut off switch in line with some sensor or wiring for the transmission, so that it will only shift out of park when the switch is on? If not, is there anywhere else I can put a cut off switch that would be hard for a car thief to figure out?

 

All I see you doing is adding another failure point to inconvenience yourself more than anyone else. What is the likelihood that your truck would be stolen?

As far as what to tie into... The shift lock solenoid could be tied into with a switch that would prohibit it from releasing the shifter. If you want to get that far into it, I suggest getting yourself a factory service manual and start studying.