I have an 84 D150 shortbed with a four in the floor and 360 under the hood, and whenever I get on the highway around 3rd or 4th gear if accelerate fairly quick the whole truck starts shaking real bad, but if I ease the gas down real slow it helps the shaking a little. Any suggestions on the problem?

 

have you had the tires balanced?

 

Sometimes if you've Been in mud then a piece may get stuck inside the rim, this has happened to me once or twice, it doesn't take a very big piece to give a rough ride

 

I would say very likely either an out of balance or an out of round tire. Was it parked for a long time? Older tires maybe? Im going through the same thing on my '85 1 Ton. I rebalanced my tires because of the same symptoms and one of the tires was visibly out of round. Like watching an egg roll across the counter lengthwise as it spun on the balancer.

 

Have you hit anything lately like a pot hole or a large rock or a ditch? Have you looked at all the tires for bubbles or defects?