2003 F150 4X4 4.2 377,000kms

My 2003 F150 with 4.2 threw codes p1131 and p1151. I did a lot of searching on here and found lots of people with the same codes but never a resolution as to what it was. I think I have mine figured out but want to hear from the experts here. No driveability issues. Truck still runs fine, have not noticed any change in fuel mileage. I know the codes mean there is a vacuum issue. I put my vacuum gauge on it yesterday when the light came back on after a few days of driving and it pulled 19 inches of vacuum. So after reading on here about the intake gasket contracting when it was cold and allowing it lo leak until it is warmed I went back out this morning when it was -18 celsius and it only pulled barely 15 inches while the truck was still on high idle. Am I correct to assume that it is the upper intake gasket. As well, I went in to my local NAPA and the only intake gaskets they list are the lower and plenum chamber gasket. Is the plenum chamber gasket the same as what I have heard on here being referred to as the upper intake gasket. Is this what needs replaced. I will be doing the work myself. Any special tools or tricks. Thanks to anyone who can help me out here.

Marty McLeod