If you have different tyre sizes the transfer case has to slip to make up the difference.
In the R33 and later 4wds there is preload on the clutch inside the transfer case so it doesn't run free like the r32 4wds do. This is why you can not safely just pull a fuse and drive in 2wd with anything later than a R32.
The reason this causes shudder is because the clutches in the transfer case grip until the load between front and rear builds and then slips. This happens over and over very quickly causing a shudder. It can happen anytime, whether under load or not, it just depends at what point the clutch wants to grip/slip.
The shudder can also be caused by air in the system, get it bled out by your mechanic
My car has a shudder under mild load and is caused by air from a leaky bleeder and gasket, it is on the list of things to get fixed.