All ecu's from 96' if you're referring to US domestic market. 02' to 06' you'll get varies levels of support in OBD2. However if you get something that supports Subaru Select Monitor, you should be fine.

I recommend OBDKey, it'll work with your GC and should work with your Liberty. Email them first though to confirm.

Check out below for more.
http://www.perth-wrx.com/vb/loggers/...sm-obdkey.html