The engines in pre 2001 are 2.5TD and are almost bullet proof the later engines have an annoying habit of kicking the number three conrod through the block and the shells wear out (too soft) and the big end bolts are not considered up to the job. The seal between sump and oil pump is also suspect.
See chris vince write up on http://www.nissan-navara.net/ Lots on this site about d22 and associated woes.
Mine has been modded to avoid these failings and runs like clockwork with over 115,000 miles on the clock.
Once modded they are a great reliable workhorse.
Tony