01 duramax cooling system issues

[foxhole]

i have a 2001 Chevrolet silverado crew cab dually 4x4 with the duramax and the Alison. it's at about 210,000 kilometers. was driving down the highway a few weeks back and the low coolant light came on. initially it seemed to be just pushing coolant out the overflow so i replace the rad cap. i also noticed a few drips later on the front drive shaft at the transfer case and bottom of the bell housing and was able to trace it up as far as the exhaust crossover but couldn't actually see where it was coming from.

it keeps blowing coolant out of the overflow and it seems a little weird when i fill it up and bleed out the air. my first thought is head gaskets but i ran 6 combustion leak test in the course of an hour today and couldn't get one positive test. when i left the shop i had filled it and bleed the air and when i got home the light was on yet it hadn't ejected any coolant from the expansion tank and there were no drips underneath, almost as though it burped a big air bubble out and the level went down. I'm almost stumped here, i don't really want to pull the motor and re gasket it without a positive test result for exhaust gasses in the coolant but am not sure what else could create this condition.