replacing main bearings

I have a 1990 xj with the inline 4.0. My engine started knocking not so long ago so i had the connecting rod bearings replaced and guess what?? the knocking is still there well now we have decided to replace the main bearings and i need to know should i replace the crank also? and if anyone has done this before how hard is it to do and how long does it take? and is this a professional mechanic job or can I do this myself to save money?
any help greatly appreciated

You might want to get a second opinion on the main bearings being the cause of your noise. These engines are more known for piston slap and loose converter bolts that cause lower-end noises.
If the mains are indeed bad, check the crank for scoring and out-of-round, which would dictate a re-grind.

Also, if the mains are bad, I would expect to see very low oil pressure.