Diff
 

Ok i have a 91 GSL that has a JDM TII motor and trany but had the stock diff but i have found a 87 TII diff and i was wondering if i could just change out the whole thing i also have the drive shaft.

diff
 

there should be no problem with this its an easy swap the only diffrence is that the S4 rears were clutch type which most appear to prefer anyway. The S5 are usualy viscus type if there limited slip. Make sure u are using the larger turbo half shafts as well the na half shafts are smaller and will not work well 4 turbo. Drop the entire rear carrage and change out the rear and bolt it all back as one assembly then connect the drive line and you are done!

thank u i know all that but i did not know if i could just take out the whole thing and change it out with the 87 on if not i was going to just buy a new ring and pin for it but no i can just do that

yeah a ring and pinion won't solve your problem. you need the turbo companion flange the turbo drive shaft won't bolt up to a na companion flange .