^Thats the same outcome I was getting with our 01 4L60.
It worth looking into as well as aftermarket lockup switches if you determine the same.
I'd think you basically never use D (overdrive) pulling a significant trailer with that rig unless downhill or flat with a tailwind.
Trans cooler may not be worth investigating. The cheap easy button is an external cooler of your choice. Simple and effective.
Also research aftermarket lockup switches. I think they exist for a good reason, although I'm not very knowledgeable about LS 4L80 combos. I do know tha the same combos a few years newer 08 and up, pull good size trailers through the mtns at work without overheating. So something is up with yours.
