Have a 97 250t legacy that l bought on the cheap. Just to let you no first l am a ex mechanic. So l do not wanna hear it, wheel balance, tyre inflation basic answer stuff. the reason I am asking is because I am a lazy prick and cant be bothered jacking the car and spinning it up to get the shudder if it a known problem on subaru is that someone can point out to me. l will let you no as far as l have gone and other potential causes:
1.i have tried 2 sets of wheels with no change to condition
2. checked handbrake it not bineding and also checked rear drive shafts at same time. All fine there
3.put fwd fuse in and problem still exists
4.done full lock test to check trans 4wd centre diff, fine
5. tyre pressures weights and alignment fine and there is no pulling through steering.
6. no wheel bearing play or tie rod play
7. brakes are not binding
now the area that l have not checked and also suspect where the shudder may be coming from are the inner cv. these will need to come off for the boots to be replaced as they split and the previous owner lived on long gravel road so possibly the joints have grit and other shit in them. the other area l thought it maybe coming from is the main drive shaft hanger bearing but l have not been under the car to check the condition of it. Just to sum up the shudder happening upwards of 50kmph at its worst around 80kph then almost gone at 110kph. it is not a light shudder it is moderate.

My wife had a similar problem with her Legacy. It ended up being the control arm. l am not sure if it was the upper or lower. Hope this helps.