The full time 4wd can be used all of the time hence full time. But the part time mode is to be used only on slipery surfaces like snow, ice, dirt, mud, etc... But it should NOT be used on dry pavement since there will be a lot of strain on the mechanical components especially during turns.
In a turn, the outside wheels travel a greater distance than the inner wheels, so they have to turn at different speeds. The full time 4wd lets this happen but not the part time. Off-road in part time mode, the wheels will slip and skip due to the surface condition without strain tou your truck.
So in winter, put it in full time and forget it. Off-road, better put it in part time and 4LOW.
Hope this helps...
96 XJ Sport having the 6 cyl engine as the ONLY factory option
4" custom lift, 31" Yokohama Geolandar A/T + II, Quick disconnects, no rear sway bar