friend of mine just bought a 94 legacy wagon awd. when accelerating quickly it has a bad vibration in the rear of the car. otherwise it is smooth. only does it on fast accelerations. with fwd fuse in it quits. could a bad cv or somthing cause this

probably has a bad ujoint in the drive shaft.

there's a possibility it could be the carrier bearing for the driveshaft as well.

crawl under and grab/shake both halves (front and rear) of the driveshaft. you might be able to detect some looseness or play in it if it's really bad (sounds like it is). just keep in mind that if it's "solid and tight" that does not at all rule out the driveshaft. they can easily stay in place and appear fine until you pull them to inspect. a seized joint feels solid on the car but causes terrible things.

 

it's possible one of the rear CV's is bad. if the boots are intact and don't look recently replaced and the axles don't look new then the rear CV's are probably fine. they could be bad, but doubt it.

 

i doubt it's your rear diff by your description.

 

if you pull the shaft you'll be able to tell which ujoint is bad or if it's the carrier bearing. driveshafts (ujoints) are very common, so that would be my guess. it'll be really lumpy or by the sounds of it - completely or nearly seized.