RPO codes question

[Killain]

I have a 1990 2x4 Silverado that I'm close to finishing restoration, and it looks great. I'm trying find out the factory codes for the rear axel, I have all the codes in the glove box, but two things kinda throw me for a curve. My truck has a code 'GMC & GU6 the latter being a ratio of 3:42. Now the truck had/has a heavy duty towing hitch, but my RPO codes don't show a towing package (I can only asume it was a dealer installed item, but you would think this setup would also have a Posi-Traction rear end, but I can't find any mention of it.

Finally what should I look for in buying a rebuilt Posi rear end ? There are a lot out there, 28 spilne 30 spline and so on. The truch TBI and I bought a "Throttle Body Spacer to go under the injector throttle body. What do you think about the spaces? They clain to ad 25 horse power and 30 LBS touque, which I would love to have a little better off idle pickup. And do you folk think that besides the frame installed towing hitch, ( it is mounted with 6 x 3/8 inch bolts, three on each side / Thank you !