Low idle vacuum, fine with throttle

[rangerlocal]

So I'm having a bit of an issue here. My understanding is at idle this car should be at 18-20 inches of vacuum. I'm checking the vacuum from the brake booster line just for reference. At idle warmed up at around 800 rpm, I'm only getting 16" vacuum, but when I give it a little throttle, up to say 1500-2000 rpm or so, I'm getting a steady 20". There is no needle waver, no smoke, no check engine lights. Has a slight rough idle, but I think I just need to clean the IAC. I don't see any vacuum leaks or bad hoses, but I haven't sprayed carb cleaner around yet since I've been working on the transmission the past few weeks. Is this normal for the 3.0l? It has 149000 miles on it BTW. Thanks in advance guys!

 

[rangerlocal]

So probably talking to myself here, but I'm changing the water pump at the moment, and then I'm going to get my fuel trim xgauge programmed in my scangauge so I can monitor the trims when I spray carb cleaner around. All the hoses look good, so I'm going to have to watch the trims to check for a possible vacuum leak. I figured a leak big enough to drop vacuum that far would trigger a CEL, but I guess not. I'm hoping that's all it is.