Right now, you are coasting down hills as your TC is unlocking and transmitting virtually no engine braking to the rear wheels. In my opinion, getting the TC to lock and adding a exhaust brake should be the first bomb (from a safety standpoint) for anyone that tows heavy, especially in areas that are mountainous or lots of stop/go driving. The difference in stopping ability is night and day!
You really shouldn’t use an engine brake on totally stock trans for 2 reasons. First you need to lock up the converter (that’s where the Commander comes in) or you’ll end up cooking your ATF and gain very little in terms of engine braking. 2nd, on a stock trans/TC with a lockup device has very low torque holding capacity and will typically result in a destroyed TC clutch. Very low line pressure at idle and a single disk converter equates to very minimal holding capacity. The ATS VB you have in your truck nearly doubles the idle line pressure. When used in conjunction with a triple disk converter you will have more holding capacity at idle than you had with a stock TC/VB at WOT!
ATS has been at this for a long time and have MANY customers doing exactly what you are wanting to accomplish. I have rode in quite a few auto equipped trucks that were properly set up to run exhaust brakes - works just as good as a manual truck. You will not regret it!