Differential Gear Size

[GM Dude]

I have decided to get brave and change out my differential gears from 3.23 to 3.73 in my 2005 GMC Crew Cab because with 20" tires and towing a boat, the final ratio of 3.08 +- is just horrible.

After researching web sites to buy the ring & pinion, they show 8.5 or 8.6 and some show both together as one group. It's confusing because it looks like they are saying 8.5 and 8.6 are the same on some sites. One site states that the 8.6 will always have disc brakes (which mine does not) but do not list the 2005 for the 8.5

 

Now the question I have for the experts on this site - what size ring gear does the 05 GMC Crew have or how can I comfirm it? It's my only mode of transportation and really would like to order the parts before I tear it down for confirmation. Afterwards I will get a wait4me tune to compensate for the gear & tire changes as well as performance mods. This leads me to my next question, do I have to get something else programmed for the ABS system? If so, will they be able to do that without touching the new wait4me tune?

 

Thanks