Bog/rough running at 70mph

[Awann99]

I've been chasing a weird issue on my daily. 350/HEI quadrajet c1500. The truck will drive great but around 65/70 mph on hills especially it bogs down. The vacuum gauge will drop to around 5 from the normal 15-20 and the truck will start to shake/vibrate. Seems like it's coming from the drivetrain. There is a deep droning noise you can hear. It will hold 70 but it doesn't sound like it likes it. If I let off the gas it feels like the motor "catches up" and the vacuum gauge goes back up and the truck stops vibrating and drives better. If I'm going down a hill it will go 70 no problem. Any ideas? I'm thinking the distributor isn't advancing right. Timing is set at 12 btdc. How would I check this? Thanks guys.
Posted via Mobile Device