Ok, are you needing to know this value so you can get the correct pinion shims put back in there? You are correct on not being able to measure the preload with the carrier installed... unless there is some other set of values that take the friction of the carrier gear and bearings into consideration. It seems you could measure the preload shims and the thickness of the bearing, add them up and replace with new shims of the same size. After all, that is the way it was set up when new and everything was "fresh". I would definately contact a "rear-end specialists" to confirm though. I'm more of a "top end" kind of guy so take this with a grain of salt.
I'm guessing your diff. is like the Dana setups...
