violent jerk

It's not a self description or any form of recommendation , but when I change from 1st to 2nd gear I get a violent jerk, unless I very slowly release the clutch. I'm sure it's getting worse. It seems to be excessive play/slack somewhere. along the drive line. I just pulled the swing arm off hoping to find something had come loose, but not so far. The cardan seems ok play wise. But when I rock the splined end of the universal joint back and fourth while in gear there does seem to be quite a bit of slack....to me... I don't know what the tolerance is for that or if I need to keep digging further?
Is there anything I should do or look at to get to the bottom of this? 
Many thanks in advance.

Just to add to that. It's not just between 1st  and 2nd, but more noticeable there, it's all gears or when going on and off throttle etc.

It would help to know the mileage, but no matter.

There are 2 drive chains in the transmission which can get sloppy over time, depending on a number of factors such as oil change frequency, type of oil, type of driving etc..
The primary chain runs between the crankshaft and the secondary shaft. (see page 189) 22mm slack max
The secondary chain runs between the secondary shaft and the drive shaft (see page 191) 21mm slack max.
These are the service limit measurements in the Factory Service Manual to reference to.
Slack in the primary chain also has a big effect on engine camshaft and ignition timing since both the camshaft drive chain and the water pump/ ignition drive chain are driven off of the secondary shaft.

 

It has 36000 miles. Oh dear it often seems to be a worst case scenario with me, so I'm likely looking at a total rebuild.    Something I unfortunately don't have the time/ability or money to do. Maybe it's time for me to part ways with the old girl then. 
Thanks anyway Kawboy! 

CLUTCH
If you only notice the failure when activating the clutch, it may be that it has a problem.

If it were a primary or secondary chain failure, you would notice it when giving gas or decelerating at low revs without activating the clutch lever.

A slack in the chains above the tolerance would greatly affect the timing of the set-up, not so much the gear change.
video chain

Thanks for that KawaBCN !
I do feel the slack when giving it gas. I found being very slow and gentle on the clutch helped to hide it. You can blip the throttle and it takes up the slack with a bit of a clunk. I was hoping cush drive but no such luck.
I just had another look at it and when I rock the drive shaft back and fourth while in gear the play just doesn't sound deep in the engine at all to me.  It sound like it's just at the end of the drive shaft...could it be the bevel gears?