bang when slowing down

99 f-150 4x4 truck has a noticeable bang while slowing down with brakes applied . does not appear to do it if brakes are not applied. brake job was done in the past six months. hoping it is not transmission ,not sure if the bang/clunk is felt in the brake pedal. I haven`t driven this unit ,someone made an inquiry. Any ideas anyone or questions that I can pass along to owner. transmission appears to be shifting normally. owner seems to think noise is coming from rear of vehicle
Thanks Muff

I would check the U joints on the drive shaft. Slide under it and check for play by just grabbing the driveshaft at either end.

It could be in the rear end also.The Ford 8.8 is not a good rear end,It is a big POS and weak.Most yards sell them easily.If someone tells you this rear end is a good rear end,full of BS.

Thanks for the input guys. Talked to the owner today .He tells me that the bang he hears would make him think that something has rolled ahead and struck front of box.. Went for a quick lap around the block today and all was well,go figure . Had a brake job done and noise was there before and after . could be one of lifes mysteries but will check u joints.
Thanks Muff