Hey guys. First post as you'll notice, thought this would be the place to ask.

Own a VR Acclaim with about 180 on the odo.

There's an issue that I've had for a while and trying to establish whether it is the diff or trans.

The time this is most noticeable is going between 70-100 on a highway/freeway and trying to accelerate up to 100. I'll be cruising in 4th and even the tiniest push on the accelerator results in a violent 'clunk' noise from what feels like somewhere between me in the drivers seat the the rear tyres. The clunk happens anywhere from from once a second to several times a second. This is 100% correlated to how hard I'm pushing on the throttle. It is basically impossible to get from 80 to 100 in 4th. I have to sit in at 80 until the road basically goes on a slight decline which obviously makes it easier for the rear wheels to be driven.

There is only one solution to this problem, and that is to floor it, resulting in a change down to third, where I can easily get up to 100, then back off the throttle.

I thought definitely diff, but what makes me think it might be tranny is that the whole changing flooring it to change down to third makes the problem go away. I would have thought it would be present in any gear at that same speed. I suppose I could describe the sensation as a 'trying to grab and failing' feeling. I picture in my head two cogs trying to interlock teeth and its just not happening.

I don't have any issues when cornering (or at low speeds at all). In fact, it's ONLY in 4th. I will be sitting at around 70, trying to hit 100, will change into 4th. The INSTANT it changes to 4th I get a clunk/thud, maybe 4 per second. I instinctively now know to take my foot from 1/2 throttle to like 1/10th throttle when my car shifts to 4th.

I can see something just going boom one day.

What could it be? And what would the wallet damage be?

Cheers!

P.S Apology for wall of text.