I am preparing to install a set of motive 3.73's in my rear end and I have purchased the following:

ring and pinion
spacer and bolt kit (to adapt to my 2 series carrier)
rear end bearing and seal rebuild kit. http://www.rmsautoparts.com/proddetail.php?prod=RA320

The rear end kit did not come with any shims and I am confused as to whether or not I can use the shims that are currently in the rear end or do I need to have this kit handy? I know I need diff fluid and additive. Am I missing anything?

 

Shims are used to set the correct backlash (gap) between the gears. Sometimes you need them, sometimes you don't. It's specific to each rear. It'd be best to have them so you don't find out you need them halfway through the install.