my brother's 89 is equipped thisaway:

w250 5 spd, complete banks setup on it, 33x12 agreesive tread tires, new borgeson shaft, new steering dampner, recently aligned, recent 'going over' underneath for anything loose.

The truck does not drive very straight, but the worse thing is that when you get off the throttle quick the truck does a little dance, does it to a lesser extent when you stab the throttle too.

The truck has airbags at all 4 corners, deflated at the moment. It's got some kind of overload/slapper bar like setup on the back too, we're wondering if those could be preloaded messing the truck up?

any ideas?

 

Have you checked out the spring bushings? A failed spring bushing on one side could cause this. When I drove school buses, I had the trailing leaf on one side break, the back of the bus would move side to side about 6 feet depending on whether I was on the gas or coasting. Fun ride! Only had to drive about a mile back to the shop, luckily.

 

could be a broken leaf spring centering pin

 

well, i found most of the problem, the spring u bolts had backed off again! new ubolts/hardware will be coming shortly