I've got a 99 GNC suburban 4x4 6.5diesel and transmission is gone I know where I can get a 4l80e with transfercase reasonable priced it's out of a 2003 chevy1500 h
Pretty sure you mean 4l80e since there is no 4l90e. If the 03 is a 4l80e, it will work. The converter will be VERY different, and being it's an 03 the nsbu will probably be different. Otherwise it will bolt in and work, only other difference is the bolt hole at 12 o clock on the 03 may not line up to your block, but I don't believe 99 used that bt anyways.