I have a 2000 Yukon XL with 121K miles that has had a hard starting problem and very low idle issue for some time. After reading AF posts I replaced the IAC with no improvement and then checked the Fuel Pressure regulator and discovered it was leaking. I replaced it and the problem was solved. However, the FPR that I bought at Advance Auto Parts caused a significant vibration in the fuel rails that I suspect is due to the pressure being too high? This FPR had no adjustment screw and I considered it to be defective and exchanged it for another but had the same vibration issue. I them installed a GM part but it is now starting harder again. I am hoping that it needs to be adjusted. What is the procedure and tool needed to check the fuel pressure? I assume that I can adjust the pressure with the set screw? I'd appreciate any assistance.

the first thing I would check is the vacume line that attaches th the fpr if it has a kink or a clog because if youdont have enough vacume ihe regulator wont work corectly

For starters you need a fuel pressure gage. Make sure you have a fresh fuel filter in there also.