Hi, Anthony. I gather from your post that your D6D is a powershift machine. If so, that clicking is normal. What is happening is that when you select a different gear, the transmission dis-engages the speed clutch pack and the direction clutch pack that were engaged and engages the new ones. The 'click' is the clutch packs being engaged.
Cat Powershift transmissions are designed to handle full power, full speed, opposite direction changes without self-destructing immediately. The de-celerator is provided for people who want their machines to last a little longer, so that they can throttle back, make their change and throttle up again without having to forever reach for the throttle lever when changing.
If you are simply stopping, there is no need to go from second to first to neutral. Just push the lever straight through first to neutral and apply the brakes if needed. If changing down to slow down or to ease the load on the engine and drive train, simply kick the decelerator down, make your change and let the decelerator straight back up again. Don't hesitate or try to do it gently when making changes. Push the lever straight through from one position to the next.
When descending steep slopes, either forwards or backwards, use first gear as the Cat Powershift system provides substantial engine braking effect in first but the load will over-run the engine quite quickly in second on steeper slopes.
I would not advise dis-egaging the steering clutches when the engine starts to lug down. If you had a direct drive machine (same engine), you would be probably surprised how far you could lug that engine down and have it pick up again smoothly when you lighten the load. The steering clutches are designed for steering, not for starting the machine.
When you have a fair to heavy load on the blade, steering in small increments is the golden rule. Trying to turn sharply places heavy loads on the track that is still driving, on the undercarriage in general and the track frame diagonal braces and sway braces on the blade in particular.
If you have to make a sharp turn with a loaded blade, it is better, where possible, to push the load to the point of the turn, maybe push several blade-fulls there, then get behind them facing in the new direction and push them again to where you want them.
When turning under load, you very often won't need to use the steering brakes as the load will do that for you. The exception here is when you have one side of the blade loaded and are trying to turn away from that side. You will most likely need to use the steering brake then.
I operated one D6D in my life for about 3 days about 20 years ago and I can't remember if it had inter-connected steering or not - that is clutches and brakes both actuated by the steering handles, clutches in the first half of the movement and brakes in the second half. If this is the case, just pulling the steering handles is enough for most steering needs, assuming proper adjustment of the clutches and brakes. You may use the brakes pedals for very sharp turn, to lessen the loads on your arms and hands or for stopping or slowing.
If your steering is not inter-connected, you will need to use the steering handles and brake pedals in conjunction with each other to make your turns.
When travelling downhill in either forward or reverse and the load is pushing the engine, you can do what is called 'cross-steering', pull the clutch on the side that you want to turn away from and the load will push the tractor toward the side where the clutch is still engaged because of the engine braking effect on that side. DO THIS GENTLY until you get used to it. On steep slopes the results can be DRAMATIC, to say the least.
Hope this helps.
You have a wonderful day. Best wishes. Deas Plant.