Why does my truck overheat only when 4x4 engaged?

What is your question?

Temperature stays good when driving on regular road with 2wd. Once i engage 4wd high or low it starts overheating after awhile.

What makes this problem better or worse?

It doesn't overheat when driving regular 2wd.

How long has your 1993 Toyota Pickup had this problem?

1 week