Hi Ahansel, I have a similar problem w. my '99 2wd C2500 under the heavy load of my truck camper when passing on steep hills. The truck shifts into some sort of safe mode. It only did this w. the TC in the bed and on steep hills. Pulling over to stop/start again always reset it back to normal (big pain in the a$$, though).
I changed the tranny oil and filter to no avail. I posted here and was told to bring it to a shop that had a good scanner, which I did. Turns out engine codes (P0130s and P0150s) showed bad O2 sensors and the Mass Air Flow Sensor (MAF). There were no 700s codes to indicate a problem with my 4L80E, only the emission stuff codes above. My grizzled old tech said he was sure that was the problem (didn't even charge me!). Man, everything is interconnected with these trucks! But I digress . . . .
I have replaced the O2 sensors (mine has 4, arrggh!) and am waiting for the GM MAF to arrive. It's already running better and I can't seem to replicate the problem with the bed empty despite REALLY trying. Once I replace the MAF and re-load my 4,000 lb. TC, I will update my post.
Keep us posted on how yours turns out too. Thanks! P