99 blazer posi 8.5 inch rear diff and says 3.42 gears
 

My quiestion is with the posi does that change the pinion gears size at all Im about to replace a pinion bearing and dont want to order the wrong one 8.5 inch rear and the codes said it was a 3.42 gear

Following.

If you have the GM 8.5" axle, then you need bearings for the GM 8.5" axle. The type of differential involved does not impact the bearing selection.

Proper identification of the axle is key though so be sure that it is the 8.5" axle that you have before ordering.

8.5
 

I'm actually positive it's the 8.5 and I spent the last week and a half studying off this forum and I really appreciate all the help I found on it and that's why I created an account and I appreciate you letting me know that I ordered one for an 8.5 but I want to buy it until I was certain I had the right one thank you for your advice

Replacing the pinion bearing is going to require that the gears are set up again with the proper mesh pattern. You will also need a new crush sleeve. This requires special tools and know how.

Pinion re build
 

I am fully aware of the tools I will need and I'm bringing into my friend's garage where I have access to a bearing press and I'm aware of the fact I will have to set the preload on the pinion nut to make sure my pinion gear is in proper alignment with my gears I've been researching and studying for last week and a half to do this job cuz it would be my first official rear end rebuild but I've worked on vehicles since I was 12 ...and I have a backlash tool Im borrowing from a friend and I have grease so I can make sure the pattern is correct when I spin my gears through.... I actually have another question I'm also doing my power steering pump which is easy compared to the rear end and I found a module under the cowl right by the oil filter and there's actually two of them but this one's Fallen just dangling by The Wire and was originally riveted on I'm going to upload a picture if you could tell me what this is it wouldhttps://cimg0.ibsrv.net/gimg/blazerf...1b0b52233e.jpgWhat is this?
be much appreciated

Pinion kit
 

https://cimg9.ibsrv.net/gimg/blazerf...2f938b28df.jpgAs you can see I have all new bearings crush sleeve grease pinion seal axle shaft bearings Etc ignore the gasket maker that came with I have a brand new gasket and there's a new gasket on it right now cuz I didn't want my rear differential leaking anything more than I could help
https://cimg1.ibsrv.net/gimg/blazerf...6d200ba019.jpgThe Fel-Pro box is a brand new gasket And there's a brand new gasket on the truck right now it's still leaking out the pinion seal but it slowed it down makes it so it's drivable and it isn't hurting my gears I keep an eye on how low it gets I'm doing this job in like 3 days so I should be okay I drive my car like a grandma anyway

I believe that component in your picture above is one of the airbag sensors.

Last year I changed to a G80 diff, new ring & pinion gear, new carrier bearings and new pinion bearings. For the pinion bearing set up I sanded out the inside of the old bearing so I would slip on the pinion shaft, tightened up the pinion nut to 0 play in order to calculate the shims needed for pinion depth. Once I had the pinion depth correct I installed the shims, pressed on the pinion bearing. To get the running torque correct I could not crush the sleeve by myself with hand tools. I ended up with a pipe wrench and cheater pipe against the floor to hold the pinion shaft from turning, and very carefully used my impact gun to slowly tighten the crush sleeve and kept checking the free running torque with my inch pound beam style torque wrench. I ended up right at the correct value. (26 inch lbs) IIRC.

Airbag sensor yes
 

Yes it turns out it is an airbag sensor and I'm going to zip tied up or something looks like it was originally riveted or some kind of plastic Clips the rotted out I had no lights on my Dash so I definitely want to put it back and make sure I don't end up with lights on my Dash lol and I just ordered some nice torque wrenches so I'm not worried about that I have a cheater bars also and pipe wrench is also and all kinds of stuff for the last year-and-a-half but I have stepped it up to get the torque wrenches big jackstands big boy Jack LOL etcetera and I've been doing my reading every night I know what I need to do to get this done correctly I'm not trying to do this twice thank you for your input I appreciate it the more I know about how it's been done before the better