Be aware, however, that your title to this thread is a bit misleading.
Your V8 truck (w/o HD towing package) is rated for 3500 lbs max towing, which is Class 2. Adding trailer brakes is not enough to get you a class 3 rating. That is, adding trailer brakes alone does not increase your max tow rating.
The good thing is that an axle ratio upgrade was not part of the HD tow package starting in '06. All V8 trucks are good with the stock 3.55 axles, because of the 6-speed trans. HD tow package did not include any additional trans or engine cooling, either.
But to get past the 3500 lbs rating, you also need (in addition to the trailer brakes):
-Class 3 receiver. Note that your 1¼" Class II receiver is a welded member of the structural frame. You can't/shouldn't attempt to remove and replace it. You would need to install a bolt-on Class III receiver underneath the Class II bumper unit. If you intend to go beyond 5000lbs loaded trailer (Class IV), be sure the hitch indicates what you need, up to 7400lbs (which is the max rating for V8 trucks with HD package).
-WDH if TW exceeds 500 lbs (a 4000 lbs trailer could easily exceed 500 lbs TW).
Give then above info, I do not understand why they did not include the HD tow package on all V8 trucks:
a. There can't be any difference in price between the 2 welded receivers.
b. WDH is a user add-on either way.
c. No difference in axle, engine, or transmission setup/cooling.
The only additional cost is the 7-pin/4-pin combination connector. Should have been standard on V8 models.