What type of rear end do i have? LOL

[gt350hr]

   hurlbird,
       If you are going to replace the seal yourself , take a feel of the rotational effort "before" you loosen the pinion nut. This is so you can "carefully" tighten the nut after replacing the seal itself. That is because ( unlike your Shelby) this diff has a "crush sleeve" and it is VERY easy to over tighten the nut and destroy the bearings. A slight increase in drag is OK but not too much tighter nor any looser that it was/is. Your Shelby ( nodular case /Daytona pinion retainer) is made with a "select fit" , solid spacer and can't be over tightened.
  More than likely your seal is simply worn too much and the yoke is not grooved. Don't add the seal saver unless you can actually feel the groove.
   Randy

[KR Convertible]

I usually clean the nut and the threads with brake clean, use some dyekem and scribe a line on the shaft and the nut.  Take a picture with your phone showing the threads and marks, so you can get it where it was.  I always use this procedure and assume a crush sleeve.

[gt350hr]

  You can do that but "I" would go just past your reference point. I go by feel because of my 50+ years of doing them , plus my Ford factory trainer taught me that in '68.
    Randy

[pbf777]

   ....................., take a feel of the rotational effort "before" you loosen the pinion nut. This is so you can "carefully" tighten the nut after replacing the seal itself. That is because this diff has a "crush sleeve" and it is VERY easy to over tighten the nut and destroy the bearings. A slight increase in drag is OK but not too much tighter nor any looser that it was/is.
 

      Follow Randy's outline or you'll be sorry!       

      To take this process a few steps further, one can use a dial-type or even a beam-type torque wrench for the "rotational-effort" reference, that is if you haven't done a bunch of these and have "the feel".  The factory "drag"/"resistance" effort as referenced is with the pinion assembly removed (5 bolts) from the pig (and this is the correct and best process), on the bench with new bearings and races, was 15 inch pounds in rotational resistance, that being after the increased break-away effort.  With used bearing components one should not attempt this number, but rather as Randy said: "a slight increase" is probably warranted to take-up for acceptable wear.         

      Should you choose to remove the pinion assembly don't toss the steel shim, as this is not a gasket to be replaced!  You wouldn't believe how many requests I've gotten from people over the years wanting a new gasket, and had thrown the odd one away!  I would just pull out the stack of shims of some twenty different thicknesses and say pick one!     
       
      Also the pinion nut is intended as a one time use item, as it is a "crimp" interference fit nut for retention, and often the flange face gets damaged (galled) which makes reuse also a bad idea.  But if the nut appears to be in good condition, I would recommend the application of "Red" Loctite on final assembly.

      And to be clear, no matter what your buddy or neighbor, who knows allot about cars says, there is NOT a torque value for tightening the pinion nut!       

      Scott.