Terrible squealing noise when braking in reverse/ less terrible in all forward gears except 1st

[Nathaniel Corean]

I've got an '88 1500 4x4 short bed, 350, 4l60/700r4, np241c 10 bolt 8.5" rear diff. The other night I shifted into reverse to back out of a parking spot and it lagged a split second and then clunked into gear really hard. I drove it a couple miles home with no problems. An hour later I went to drive it somewhere else and under heavier acceleration I was getting a pretty loud squeal from somewhere underneath the truck. The engine was running fine. When I stopped completely it would stop. If I put it in reverse it happened constantly and was borderline unbearable. Kind of sounds like stripped gears that are still just touching each other. I immediately idled it home and no noise the entire way. Once home I put it up on jack stands and ran it through all the gears and 4x4 ranges. It seems like O/D at idles it spins the tires at about 15mph on the jack stands. If I let it get up to that speed and brake hard it will squeal as the tires stop and then stop the noise once they are stopped. So it happens for basically a split second since it stops the tires basically instantly on the jack stands. If I brake before getting it up to that 15mph it won't do it. This will happen in all gears except 1st which I have to gas it up to the 15mph, but it still won't squeal when braking. Same for each gear in all 4x4 ranges. Obviously in lo it needs a lot of gas to even get up to the 15mph in the lower gears. Reverse is different in that it squeals constantly if I hold the brakes on but not if I let them off and allow the wheels to free spin. On the ground where the wheels don't just spin freely it squeals constantly in reverse. With the transfer case in neutral it won't make any noise in any forward gears but makes a slushing sound like something is being stirred up in reverse. Transmission fluid is filled perfectly, and I drained and refilled with new fluid both the transfer case and rear diff. There is no front drive shaft but the transfer case is spinning appropriately for all gears and ranges. I can't figure out where the sound is coming from or what is wrong. I think it's coming from more towards the front, transmission area, wife thinks it's more from the rear like the brakes or rear diff. Rear diff was low on fluid when I drained it, but all the gears inside where in good shape with no signs of wear. Any obvious things to check or known issues? I've only had the truck a couple months and never had one of these before so not very familiar with them. I can post a video if it'll help so you can hear the sound.