Probably slipping diffs and/or slipper clutch.
The basic test is to hold the spur gear and one wheel on an axle still* and then try and turn the opposite wheel. It should take a decent amount of effort to "slip" the wheel, and it should be the slipper that slips first.
Adjust/rebuild according to the manual if either slips easily.
The diffs should also have a smooth but not free-spinning action, and the front diff on a 4wd should be set a little tighter than the rear.
The slipper in a 4wd is basically only needed to protect the drivetrain, traction and wheelying is not an issue.
*needs a bit of contortion but can usually be managed with a thumb on the spur and a hand/arm/body gripping the wheel
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