Yeah, I just put a Disco I TC in our defender too. As was said above, you'll likely need to swap the speedo sensor from a D-90 TC. You'll also need to remove the shift lock solenoid. It's right on top of the TC and you remove the four bolts holding it (it has a wire coming out of it) and remove the solenoid from inside the housing (the same as permanently releasing the solenoid) and put the housing back on with some silicon sealant to plug up the hole left from the wires. Works great!
Mom: "Boy, you got champagne taste on a beer budget."