For what it's worth...
Gear whine or howl is from the improper pattern setup. Once everything is removed from the rear differential housing, or the 'pumpkin', the pinion is set up first. The tricky part is that you have one time to set it up correctly, or your screwed. The crush collar that is included with the gears is only made to be 'crushed' once, and that is when you are setting up the pinion. Torque on the pinion is measured in inch-pounds, not foot-pounds, and most mechanics and shops do not use an inch-pound wrench to set up the pinion which leads to whine or howl because the pattern and depth are wrong because the pinion was never set up properly.
Now, I'm not saying that those guys don't know what they're doing. What I'm saying is I don't really buy into the whole Pro 5.0 sucks. If Pro 5.0 sucks, MM&FF wouldn't be putting Pro 5.0 stuff into their project cars. I know people on this board will have differing opinions, but I've been around too long to believe that all Pro 5.0 sucks. FMS has a better reputation, however I don't believe that their products are any better. If I were you, I would go into the West Coast forum and ask around for a reputable shop that does gear installs and see if those people have a gear whine problem.
I thought FMS made a 3.90? If you can't get an FMS gear, get a Pro 5.0. If you find a good shop, you shouldn't have any problems.
Joe