Juice,
The transfer case does not determine which speedo gear you use. You need to know what size tires and what gear ratio you have.
I have a 88 XJ with the AW-4 Transmission. I read all automatics came with the 3.54 or 3.55 gears, seen it both ways. I have LT235/75R15 tires, for this application the service manual says to use a 33 tooth gear.
My XJ came with P205/75R15 tires stock, it came with a 35 tooth gear which I changed out when I went to the 235 series BFG AT tires.
I have a mechanical speedo cable and sure you do also. I purchased a 31 tooth gear in December 2002 to correct my speedo. I found out I got the speed to agree with my GPS but I was racking up more miles on the odometer so last time I changed oil I put back in the speedo gear.
I would go back to your dealer and say "I need a 33 tooth gear for my 84 XJ" if necessary you may have to say "it should be the same as an 88 XJ". My dealer has all the parts info for my XJ, yours should also have the info.
HTH
Martin