BAD vibration what could it be?

I have a 1985 GMC C2500 conversion van with the 305 4brl. I just bought it with 100k miles,When its in nuetral and you goose it a lil' it has a little vibration,HOWEVER,When you're in drive and you accelerate it has a BAD vibration..I've been underneath the van and inspected the drive shaft,It's as straight as an arrow,I've replaced the harmonic balancer...can anyone help me here?