Idling engine while towing questions

My 03 Jeep Grand Cherokee with 4.7 l V8 and auto trans will go to reg idle rpm at highway speeds when slipped into neutral. My 04 Silverado 2500 HD with 6.0 gas 4L80E trans will only let it go to regular rpm idle if road speeds are low. My 04 Silverado HD with Duramax and Allison is another story all together now I am not sure exactly what happened maybe someone can answer my question, one night while out of town working me and my brother were out looking for some back roads trying to have some fun and for some reason I got going about 50 or 60 and put the trans in neutral and let it cost no throttle and after the truck slowed to around 15 or 20 mph it straight to up just about locked up now I'm almost 100 percent positive it was in neutral my brother was kinda thinking more guessing I put it in reverse this was in my younger dumber days when I did used to do a little buzzed driving but it was maybe 2 or 3 beers at most over 2 hours period so that is the small chance he could be right. I am pretty dang certain though it was neutral just out of curiosity has anyone experienced this?