Transmission or transfer case?

[Mafian]

After traipsing through many threads on transmission and/or transfer case issues I couldn't find something that fully matched up with my issues so I thought I would start a new thread. I have the E53 diesel with ZF 6 speed.

My issue: Under medium to hard acceleration (through the lower gears especially) there is a slipping feeling as well as a mild shudder, first gear is the worst, it doesn't really change with the increase of rpm and feels more like a 'load based thing'. This is sometimes accompanied by a rougher than normal gear change between first and second but not as bad after this. On low throttle openings everything feels normal. Under higher loads at normal cruising speeds (climbing steep hill/bridge) whilst the revs don't move much more than normal/expected (no real slipping feeling) you can sometimes feel a slight shudder ie drive is not as 'smooth' as normal. The difference doesn't really change much between cold or warm. Sometimes when going up our steep driveway you can feel a slip before it 'takes up' as normal. This is rare though and the torque converter still 'takes up' as it should at 1,200 rpm. There are no obvious, abnormal noises, other than the ones in associated with the shudder.

Background: Car has done 138,000km. Mechatronics replaced 40,000km ago (it went into limp home). When this issue first started happening I had the transmission serviced (with oil change) and the adaptation program reset. No fault codes have come up at any stage. Transmission specialist (they do a lot of ZFs) couldn't really see anything obviously wrong inside when looking and said the fluid whilst a little darker than normal was well within specification/expectation. This service didn't really change things much.

Basically I thought I would put it out to the community if anyone else has experienced something similar and what was done to repair it. I am just looking at possible ideas before taking it to a service department/specialist for further fault finding and spending any more than I have to trying to fix it!

Any assistance is appreciated. Sorry for the 'war and piece' novel!