HELP.......Truck has major shudder.

[Mccoleman830]

I fired up the truck today to discover that my 78 has a major shudder problem. Trying to diagnose the problem has left me clueless. It shudders extremely bad when in reverse, and all drive gears. When placed in park and neutral it still has a slight vibration. I tried to drive it down the street, but while a stop light you have to keep your foot on the gas pedal or it has a violent shake. Any advise where to look at would be great. 1978 GMC K-15 350/350. The only thing ive done lately was get a fill the tank with fuel. I thought it might be bad fuel, but switching tanks from right to left doesn't help at all.

Thanks.