I have heard that GM made a differential at the end of 1985 for Irocs that had a 3.42:1 ratio open rear end with the housing #9PE that had straight cut gears. I looked in my repair manual for 1985 and found 23 different rear ends listed for 1985 that all started with the #8 and no #9PE. My manual must have come out before the #9PE series rear ends were produced in late 1985. After that, I looked on line and found that they did make a #9PE rear end in 1985. I found it unusual that the 23 other rear end numbers started with the #8 and only one with a #9PE. Could this be a unique rear end with the straight cut gears I've heard about? Straight cut gears tend to be louder but much stronger. Just curious if anyone else has heard about this?

What gears are straight cut? Are you talking about the ring & pinion gears? If you are then you need to take a look at them. They aren't straight cut. They can't be. The way that the housing is designed they can not be straight cut with the pinion offset lower than the center line of the ring gear.

What do you mean by closed rear end?

Straight cut gears are used in the transmission....

Bevel gear
Main article: Bevel gear
A bevel gear is shaped like a right circular cone with most of its tip cut off. When two bevel gears mesh their imaginary vertexes must occupy the same point. Their shaft axes also intersect at this point, forming an arbitrary non-straight angle between the shafts. The angle between the shafts can be anything except zero or 180 degrees. Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called miter gears.

The teeth of a bevel gear may be straight-cut as with spur gears, or they may be cut in a variety of other shapes. Spiral bevel gears have teeth that are both curved along their (the tooth's) length; and set at an angle, analogously to the way helical gear teeth are set at an angle compared to spur gear teeth. Zerol bevel gears have teeth which are curved along their length, but not angled. Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears. Straight bevel gears are generally used only at speeds below 5 m/s (1000 ft/min), or, for small gears, 1000 r.p.m.[6]

Taken from http://en.wikipedia.org/wiki/Gear.

So what does that have to do with the rear end in your '85 IROC?

On heavy trucks. Unless you put a jerico in a camaro isn't going to have them.