You bring up some good points here. First of all, the "D" height measurement is really just like the "covering" of all bases when working on a vehicle. GM, as most doctors, always seem to "check-this" and "check-that" before and after repairs. Like, disconnect the battery before changing the oil. It is just a safeguard.
Checking this measurement is listed below and you can see it is just to check for any physical damage so you don't blame you trim height on bad suspension components.
D Height Measurement
|1. ||Jounce the rear suspension of the vehicle by pushing the vehicle down and lifting up. |
|2. ||Allow the vehicle to settle and take a measurement. |
|3. ||Measure between the axle bracket and jounce bumper mount bracket as shown in graphic. |
|4. ||If any of these measurements are out of specifications, inspect for the following conditions: |
| ||• ||Worn or damaged suspension components |
As far as just one or both bags being bad, most likely they are both bad. It is not that they break or have a hole in them, it's the fact that they deteriorate. They are made of rubber, and they weather crack and become porous. If you take a spray bottle with a soapy solution and spray on the bags, you'll notice they will foam. Air is just seeping out of the rubber. That's why they stay inflated and go soft over-night.
As far as regular or heavy duty. If you're not towing anything and the vehicle is pretty much stock, I would just stick with what was on there. Original stock GM.
And as far as your shocks are concern, that's totally up to you. There is no set procedure for "if and when" on replacing your shocks. If they are leaking, then most definitely. Shocks are hydraulic and they really don't "wear-out" so to speak. Again, if you opt to replace your shocks, I would stay original GM. You'll have a dozen different choices when it comes to aftermarket shocks, along with a dozen different price ranges. All too often we get customer's who are not happy with the ride they get from aftermarket shocks.