87 5kTQ rear diff fluid change

[Sean Shoemaker]

Hello all,
I am looking to service the rear diff on the above mentioned 87 5kTQ and I was looking for any pointers.  I had plans to run by the dealer to pick up the requisite fluid.  I thought I would do the tranny at the same time.  Should I be getting the GL-4 or the GL-5.  I am currently thinking the GL-4 per the Bentley.  Curiousity, what is the difference and are there any pitfalls or short cuts I should be aware of when doing this.  As always thanks to everyone for your time.  I am out to make big things happen tomorrow to get my wagon ready for reinspection.
-Sean

'87 5000 TQ wagon