if you look around the league at 3-4 DTs.. the starters arent exactly all late round picks. There are as many first rounders as third and 4th rounders it seems. And my all time favorite 3-4 DT.. Casey Hampton of the steelers.. was a first round pick.
yah its possible to get a good DT in the later rounds.. but that doesnt mean that you should pass up a good one in the 1st or 2nd round.
As for our pass rush.. one of the biggest problems with our pass rush is not a lack of talent at OLB.. its our Dline. Our dline is not just there to stop the run, they have to tie up blockers and free up the OLBs.. and they have to provide a bit of pressure up the middle themselves, to flush the QB out of the pocket whenever possible. So, improving our Dline will improve our pass rush.