If you're running the AEM EMS the stock O2 sensor becomes redundant so you can mount your aftermarket one in it's place.

 

As for diff replacements, just did one a few weeks ago. Need a jack, stands and basic tools.

A full aftermarket solution involves a Nissan diff, custom driveshafts and mounts and cost something like $5000. See Inline Pro.

I cheaper solution is an OEM diff rebuilt and reinforced by PuddyMod. He does a whole load of things including stiffens up the pinion sleeve and end caps resulting in reduced flex (and that's what kills OEM diffs). He has a whole range of options to choose from.

 

sounds interesting, im going to slow down a tad... my celica daily just crapped out on me on a daily drive home... most out of the blue thing ever... no noises, my intake was covered with a prefilter, oil was good, etc.... ive had all maintenances... just drivin, rain was heavy... once it let off i went to speed up and i heard a bumbmbmmbmb really fast like a vacuum with something stuck in it so i pulled off the interstate, slid plate back and checked intake... it was dry under the water shield... checked oil... nothing... so i started back up, and anything past 3400 rpms it would shudder horribly... so i had to leave it and have it towed... cant wait to hear what BS a toyota engine did this time