1989 Ford F150 transmission won't shift?

When in Drive (automatic), the truck won't shift into a higher gear. I pulled the vacuum line from the modulator valve and didn't see any fluid so I think the modulator valve is okay but not sure. It seems like there is a "sucking" sound coming from the engine. I'm wondering if the vacuum line that attaches to the Intake manifold is leaking but I can't tell how it attaches (hard to see from top and bottom of truck). Any real mechanics out there that might have some ideas because I'm just a novelist too cheap to go to the transmission shop. Thanks!