[fredjacksonsan]

Part time 4wd locks the transfer case so that the front and rear wheels turn at the same speed.

Full Time leaves the transfer case unlocked, so the front and rear can turn at different speeds.

It's important on the road, since there is no wheel slippage under normal conditions and if the t-case is locked you can break something if you're in Part time.

If you've got the Select Trac, where your transfer case is labeled:
2WD
4WD Part time
4WD Full time
Neutral
4 Lo


Then there's no issues with reliability leaving it in 4Full Time. You just might have a little less MPG.