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Hopefully i can help..
There maybe a few other things you may want to check also if they haven't already.in that check for any excess play or noticable movement at all the front suspention arms / bushes / balljoints , antiroll bar bushes and drop links...also check condition of the engine/transmission support mounts for wear that may cause excess vibration to felt through the vehicle - also check condition of both front driveshaft cv joints inner and outer joints for wear and inner bearing on right hand driveshaft for movement - seeing as you had the front tyres balanced already then it would be worth maybe just re-checking just to be there isn't any tread damage.if you turn the steering full lock and then rotate the tyre slowly observe the treads run in a straight line and don't show signs of buckling that may of been caused by bumping up kirbs/pothole damage etc..though if that has has been done already by your garage who balanced the tyres then you can disregard that check if you wish..- another likly culprit could be the clutch disc
or two mass flywheel or DMF on these sometimes can be felt under load or sitting at idle or through the clutch pedal too and the only real way of checking such things as those ie:- damage / tilt clearance / clearance angle can only be done once the gearbox/clutch assembly has been removed...so it would be worth going thouggh the very above items first just to be those have been checked before anything like the gearbox needs removed to checked further.
I wasn't aware of that in your first question...however if this fualt has only started to crop up since the clutch replacement and wasn't an issue before or similar then it may be worth checking the other areas as above anyway seing as during the clutch chnage they would of had to remove the above items..though usually with any clutch change on these it always advisable to check the two mass flywheel as above to ensure it is still in good order and passes all the checkable features...so that maybe something to maybe re-enquire about with your garage..though if this isn't a problem that is generally from the engine itself and doesn't seem to show up when the engine is rpm is increased and seems to be more of drivetrain issue then the checks above would the way forward at least..
Ok..I would be kind of surprised wether it would be the gearbox unless the diff has become faulty "re-confirm you did re-fill any lost gearbox oil"..However as you'll know the only way to check really out the driveshaft is to feel for any excess movement before eventually having to replace it to see if the problem is rectified - provided elsewhere is all good like the suspention at that side /subframe / wheel bearing as you've checked above / the tyre "maybe try swapping the tyres over to see if the problem shifts or unless try the spare on that side" - double check the braking caliper pads,disc anyway / or there maybe an front alignment issue - "if you can but must be done safely is to maybe run it in gear with the wheels off the ground to see or hear/feel any vibration or rumbling so to speak - mght be worth trying"
- Going back to the driveshaft itself it will have a uj near hub as the other end will have a tripod/roller bearings so you need to check for movement there as above or damaged/worn roller bearings or check the end nut is secure at the front hub "you have to use a new bolt" and torque it upto to 280Nm ..also re-check also the 3 bolts at the retainer plate as when fitting the drive shaft must be held in a horizontal position by the 2nd person whilst you secure the retainer plate bolts 25Nm...Obviously the driveshaft would be cheapest part to replace before the gearbox "is Condemed" so it maybe worth trying/checking that first "shaft" or have a few of the other checks done above which wasn't mentioned before..
If the side to side/up and down is from the diff then it could be worn bearings in there.there should very little/minimul play but enough to compensate for suspention movement..you could try another Driveshaft and see if it makes any improvement.or remove the one you have and check both ends..
Provided everything associated at that side has been fully checked as above - inc the wheel bearing which i know you said you have checked / then it does seem to leave the diff on the gearbox - although something you may want to re-check is the rear wheels at that side just to be sure the vibration isn't radiating from that area and giving the impression its from the front "certainly would be worth ruling out" - further to that without actually getting a feel for it myself thats about all i can think off - one thing you may want to do first before you remove the gearbox would be to see if you have any local transmission specialists that may want to just re-confirm first and see what there impression is..