Im surprised noone mentioned the possibility (far from guarantee) that the reverse band servo piston cracked, because that is a super easy fix just by taking the pan and one small cover off. I once fixed a no reverse 4l80 by just replacing that servo piston. If you actually drive the van you can narrow it down to a problem with that band just by checking for engine braking in 1st gear (manual low). If it doesn't have engine braking in low gear, then that band isn't gripping and from there you could try just dropping the pan and seeing if that servo piston is the issue. It could still be pressure problem or a burned up band (you can check the servos 'free play travel' with the cover off to indirectly assess band wear) or even a damaged case around the band anchor or something like that but i would definitely CHECK that that piston isn't broken before dropping the whole trans.
https://www.transgo.org/demo/06-178%204L80E-85E%20and%20TH400%20%20Reverse%20Complaints.pdf
