OK, SD55's reply sounds like it has merit, but I am not sure...

Here is the way I did mine, but I am not saying for sure that this is the correct way to do it.  (Keep in mind that my diesel tank used to carry gasoline in it, and I did not want to take ANY chances !!)

I drained all the diesel out of the tank, and then I filled it with water, and let the water over-flow for about 5 minutes.  Then, I did my welding while it had water in it.  After welding and after confirming that I got 'er done, I drained the water out of the tank.  Then, I put about 4 gallons of diesel in the tank and went driving like I had been drinking. (By doing this, it made the 4 gal of diesel slosh around in the tank).  Then I drained the diesel, and put in another 4 gallons, then repeated crazy driving scenario....then drained it for the second time, then filled it up with diesel for real...I have the water block feature on my tank's filter, but it did not catch any water...I got all the water out of the tank..

I, too, have heard of the vehicle's exhaust.  In theory, this should work, as the exhaust will not support combustion.  I have never done it, though, and would not want you to get hurt.  People might be hesitant to recommend methods because IF there was gasoline stored in your tank before diesel, then that could change the whole game.  What ever you do, be careful....and if you're not SURE you KNOW what you're doing; then for Goodness sake, take it to somebody that DOES KNOW what they're doing, ok?