Slipping sound on hard acceleration from standing start

I have a very trusty P38a 2.5td 1999 vintage with 104K miles which I've done 34K miles since purchase in December 2010. Has been faultless up to now but in the last three weeks I have noticed that when I accelerate hard from about 0 - 25 mph, I can hear and feel a juddering noise which, as far as I can make out, is emanating from the front nearside area. i assume this is some emerging problem with the transmission. The sound is like something engaging and then slipping and then engaging and slipping in very quick succession. Any ideas if this a problem or something to be expected? I normally accelerate very slowly and then quite travel quite slowly so I've not noticed it before. I only noticed this as I was forced to accelerate quickly across a roundabout as some lune was pulling out in front of me without looking. The sound/judder only manifests itself when accelerating hard. Let me know your collective thoughts.

manual or auto? does the same happen when you try to accelerate at speed eg. 30 - 40 50 -70 etc?

Its auto. The issue is in normal ratios not low ratios. I always leave the box set a "D" and the driving is always on the road rather than off-road. I've just tried it out and accelerated hard at 30 and yes, its the same problem although not quite as pronounced as from when starting from a standing start. Could replicate it at 50mph. Its basically occuring when the revs are 3,000+ and a heavy drive force is being appied by the engine compared to the distance traveled. I can accelerate at 50mph to 70 but can't reproduce the noise at this speed.

I would check the VCU, transmission wind up might be the cause, it will blow the diff if left. Jack up one fron wheel , box in neutral and try to turn the wheel, it will require a lot of force but it should turn. If it doesn't turn, the VCU has seized.