98′ International 4900 Cab Bouncing

Okay, so my work truck has a issue that’s been around since the dairy assigned me this truck back in the past summer. The cab constantly bounces while i’m driving. It’s not like a slight bounce anymore. It’s only the cab that bounces up and down, the rear box doesn’t really move at all. I notice it’s not as bad in the beginning of the day when I’m fully loaded than throughout the day. I sometimes for fun watch the shadow of my cab while i’m driving to see how much it’s bouncing. I’ve noticed the last 3 weeks it’s gotten unbearable. It’s now affecting my control of the truck and the steering seems to wonder anymore but the steering wheel doesn’t have any play in it. The tires are new-ish and not cupping or have any awkward tread wear. I’ve talked to my boss about it and he thinks it’s a kingpin. The mechanic’s have no idea what it is but can’t begin work until they get back to full health, one of the mechanics is on vacation. Plus, they have no spare truck’s. Truck probably needs a front-end rebuild. Springs are in good condition. The cab bounces and vibrates violently at high speeds on the highway. But when i’m at a steady 35-50 mph, she’ll throw me off my seat like a bucking bronco.