Question on exhaust manifolds and driveshafts and mt-90

Hello again my fellow Tacoma Brethren, I am still fairly new to the sight but the info I have obtained from you all is like having an online encylopedia for Tacoma's or simply put Tacomapedia, and its pricless. I recently bought a 06 double cab 6 spd TRD. I had a lot of drivline noise? and p0420/p0430 codes which we all know is Catalyic converters. So trying to find a cheap way out of fixing the problem I searched craigslist far and wide and low behold I found what I was looking for in one add, which included both stock exhaust manifolds and Y-pipe and a whole rear drive shaft and all of them had a approx 15k on them for 500 greenbacks. I was so happy. Because the carrier bearing was still good and the U-joints too. Now all the parts were from a double cab 2nd gen <2011. So when I went to swap out the shafts I layed them next to each other and to my surprise mine was about 3/4 inch shorter, was bummed but not to bad because paid 50 for it out of the 500 spent, so I took the good carrier bearing off and bought 3 new u-joints and fixed my old shaft which had a bad carrier and a u-joint. So now here is where is stand dumb and stupidfied, because i want to throw the better exhaust system on with new gaskets and afraid i will run into length issues again with it. Like stated before the parts were from a 2nd gen double cab, but not sure if it was six speed or auto tranny or if this even makes a difference and maybe that is why my shaft is shorter.....My truck shaft? Oh By the way I put some MT-90 in my tranny and that stuff is the CAts ass, is it safe to put in my Rear end. Thanks for all your help who ever responds.

 

No the MT-90 is not safe to put into a differential.

 

Thanks I just had some left over when i did the tranny but I can run redline 75/90 Gear oil correct