transmission wind up is a bit complex....
if you take an old landrover with equal size wheels going in a straight line both axles are turning at the same rpm and travel the same distance, if one axles tyres are slightly more worn , or different pressure and bulge more , the wheels have to turn at differnt rpm to cover the same distance... not possible, (unless you have a centre diff like on a later landrover) or something has to break... given perfect tyre grip it would be a propshaft joint or cv in the front axle, in practise when the strain in the drive train builds up the wheels slip a bit on the ground to release the stress...any wear in the joints produces the audible clunk as it unloads. thats why you should only use 4wd when conditions are such that the tyres can slip.easily.. not on dry tarmac.or concrete
later coil sprung landies with permenant 4wd have a centre diff so they can drive on road... some have a difflock on the centre diff to lock it for mud.
with tractors with different size wheels the gear ratio of the front and back diffs has to be different so the different sized wheels doing different rpm actually cover the ground at the same feet per second (approximatly , in practise all the tyres will actually be slipping but at different rates)
when turning the inside wheels travel less far than the outside ones... the axle diffs take care of that... but the rear axle usually cuts the corner a bit so travels less far than the front one... again tyres have to slip to take up the differance. or you get wind up .
hope that removes some of the mystery
