I was getting a shudder at the steering wheel when I accelerate to over 50mph didnt really get worse bu not evident when I'm driving round town only on faster roads...turns out this was a partially siezed rear caliper which wouldn't release.

Wheel balance is a first step.
If the shudder is worse under load and is continuous, then it could be a misfire.
The automatic torque converter clutch can also create a shudder as it come in, which is about 50 MPH in 6th, but this shudder is only there for a couple of seconds.