I had minor headaches getting the front plugs out, they were tight, but eventually came out. The transfer case was another story, they were in there TIGHT and did not want to budge, plus, they take a very large Allen wrench instead of something normal like a regular 3/8" rachet, or at least mine were Allen head. And there's no way they were coming loose just by using the Allen wrench, I had to get a short piece of copper pipe on the Allen wrench to break them loose.
After reading this thread, I'm even more fearful of the rear differential...
http://gmtnation.com/f79/trailblazer-throttle-body-rear-diff-drain-fill-without-drain-plug-pics-1179/
There's some good tips in this thread on things that might help. As for myself, I'm going to spray the rear differential fill plug with penetrating fluid a few times before I try to break it loose.