FI Ramcharger fuel tank vent problem

My 91 RC has a bad venting problem, in that when you pull a trailer up hills after a lot of driving (pressure building in tank) it starts bucking and losing power from lack of fuel. I have replaced all the fuel lines and filter, put two new fuel caps on it, checked the filler neck, and it still won't vent. After an hour of driving, taking the cap off makes you feel like you're going to get sucked into the tank. I can leave the cap loose, but I am not satisfied with a "quick" fix like that, I want the "proper" fix.
Also, when it's running there is a "whine" coming from the fuel pump, like a high-pitch motor whine. Not really loud, but noticeable. I have put other pumps on it and they all seem to sound that way. Is that normal for these early-FI trucks?
Any help is appreciated!

I had a few of those tbi rams and never heard hardly a peep out of the factory walbro pumps. I replaced a couple of the trucks pumps, only because they had many miles on them and did not want to wait for them to fail. It sounds like your evap control system is plugged at canister or elsewhere.

If the air is being sucked in it's vacuum not pressure. Pressure will blow out. There's a big difference in the two problems. Have you checked the roll over vent valve combo on the tank?

Definitely vacuum, not pressure buildup. Sorry if I was unclear. I took off the filler neck and checked the checkball and it rolls freely when upended. Vent line is clear. I installed all new fuel lines/return lines, but have not tried to blow air through them to see if any are clogged. Could that be the problem? Will the charcoal canister being plugged cause this? The thing is, if loosening the cap relieves the vacuum pressure, you'd think the cap was the problem, right? Or is something else causing excessive vacuum in the fuel system/tank?