not a classic but a gmc. anyways, my dad has a 85 gmc vendura 3500. its a big box truck, well when we slow down from like around 25 to 35 the whole truck wobbles side to side really bad sometimes, pretty violently also. my dad thinks maybe since he has a really bad rotor, like with ridges it in, when he brakes, only the drivers side pads are fully compressing on the rotor, while the passenger side cant make full contact and throws the truck around? what could it be? thanks guys.

Sounds like a bad tire.

My vote is a warped rotor (or two). Re-do the brakes and I'll be that's the end of it.