Transmission not engaging
 

So I was playing a few weekends ago and all of a sudden I was dead in the water my transmission just stopped no weird noises or anything so we got it home and I pulled the pan which had fragments of metals. So in my mind bad transmission also I ran it low on fluid. I got a used trans/converter and got it installed and still nothing new fluid filter etc. when I clear the codes and put it into drive it will throw a code 1784 could it be a electronic issue with some wiring or something or my brother-in-law said it could also be bad transfer case or just need to reset the system with a shop scanner?

What engine/trans?