[queenbu]

hold sth/sb down 1.to stop someone moving or escaping It took three officers to hold down the suspect.2.to keep the cost of something at a low level to hold down prices/wages 3. hold down a job to keep a job It's difficult for mothers to hold down a full-time job.
whilst
hold 1.to have something in your hands or arms 2.keep something in a particular position 3.to organize an event 4.to contain something 5.to have a particular job,position or qualification 6.to have a particular position in a competition etc. etc. etc.
So I guess sometimes they do have the same meaning as in 'to hold a job' or 'to hold down a job'.Otherwise the meaning is different.