Handling problem

Mate of mine has a 300tdi 98 model. The other day his girlfriend was driving it on a dual carriageway ~ 60mph inside lane. For no reason, car developed a speed wobble and in the end she hit the central barrier. Mangled wing etc but nothing too serious luckily. The handling has been vague to say the least for a while. Nothing seems loose, no noises, no excessive tyre wear, no pulling to one side.Quick poke around underneath, everything looks and feels about right. The one thing he says, if he brakes hard, he can feel the back end squirming about.
I've given him an old pair of rear shocks to try but somehow I think its more than that.
Theres all the usual suspects, track rod ends, A frame ball joint etc. How bad do suspension bushes have to be to cause something like this?
Any thoughts?

had this on mine. Ok on mine it was the trailing arm bracket that sheared off the axle, but it seems common for the bushes to go. We ended up having to replace the shocker bushes aswell. Not saying its deffo that, and i am sure someone with alittle more know how will be with you shortly

Check the rear trailing and front radius arm mounting points, they've been known to corrode, use a breaker bar to try and lever them .. any movement is a no-no ..

Also for the wobble, check the swivel torque (search for spring balance method), and the steering damper and it's mounts.

For a wobble to develop to a crash means (I would think, other than possible driver over-reaction maybe) that something's broken, not just worn .. though obviously if the steering's been vague for a while then it's wear and breakages you're looking for. I wouldn't drive it, other than testing until you find a problem and fix it ... then be careful and test drive again .. and again as necessary.

Wheel balancing rears as well had this on mine scary stuff. already done it on a previous post'

Might be worth checking the bearings, i had one go on the M5 welded itself to the stub axle instantly, yanked me across 2 lanes, lucky it was empty

STEERING DAMPER!!
£9 do it now, sorted

Thanks guys. Much as I thought its a case of getting under and having a good heave around.
Steering damper though. Wouldn't you expect to get a lot of feedback through the steering wheel? Worth changing anyway though. As you say, its an easy and cheap fix.

Check the steering box backlash. Check lower steering shaft UJ's. Then as above, all suspension bushes, steering damper, track rod ball joints, drop arm ball joint, drag link ball joint and wheel bearings. Check tracking too.