Just a quick update - just shy of 1 year later... and I am finally on my way to resolution. The miserable TrueTrac in the rear of my 97 D90 is coming out soon. It will be replaced with an ARB. The TrueTrac in the front has been silent since day 1. I'm leaving it there. If it causes any issues with steering this winter (on snow or ice), it will get replaced with an ARB immediately. Some people say it will, some people say it won't. I will know one way or another in due course.
The rest of this post is primarily directed at anyone who is new to this discussion (like I was) and needs some answers. Learn from my mistakes!
There is SO much good advise in this thread and I wish I had the ability to go back in time knowing what I know now. My current knowledge on this subject makes previous suggestions so clear - so easy to understand. At the time I started this thread, however, I just didn't get it. Boy do I get it now.
I have spend hundreds of hours and thousands of dollars chasing what could have been resolved with a properly built DC rear shaft and adjustable rear training arms (my truck needed both). BOTH of the suggestions to do that were pointed out in post #2 and post #4 of this thread. If I really wanted to knock it out of the park, I could have bought the OEM 3rds for $500 and been stoked.
The upside has been the 4.11 gears. They really do work well for me in the foothills of Colorado. But... my speedo is way off now. Had I known... ugh.
If you find yourself reading this thread while getting ready to lift your D90 SW or you're here trying to figure out how to fix the same issues, I offer you the following opinions:
Stay away from TrueTracs in your Defender unless you are okay with a great deal of ratcheting and clunking noises. I have heard some people say their truck is quiet and others who have a great deal of noise (like me). In fact, the TT in the front of my truck has been quiet since day one but the rear has had significant issues - some related (in my opinion) to the set up and some (in my opinion) to the nature of the TT design and the design of the D90 drive-train.
Daily drivers should seriously consider ARB lockers because they're open diffs unless you lock them when they need to be locked. If you think the extra few hundred bucks for ARB diffs isn't worth it, consider the value of your sanity. If you want a quiet drive train around corners and backing out of parking spots, skip the TT's.
I am hearing (from some reputable vendors) that about 50% of D90 station wagons that get lifted 2" or less work just fine and that the other 50% need adjustable training arms, a DC rear shaft, or both to resolve the pinion angle problem. In my case, both were required. Plan in it and you won't be caught off guard.
Finally, I suggest you listen to the people on this board, if you don't understand, don't be afraid to look like an idiot by asking the same questions until you do - or you may look like a much bigger idiot for failing to take some very good advise.
Ask me how I know.