I have a '90 F-150 that shakes when accelerating between 1000 and 1500 rpm. The shaking is pretty intense when in gear, but still can be felt when in neutral. I replaced plugs and wires, checked motor mounts, and checked the fan and pulleys. All seem fine. I do hear a chirping coming from the area where the tranny mounts to the motor (flywheel area.) I'm not sure if this is related or not, but it is notable. I plan to pull the cap off and inspect it as well as the panel to check out the flywheel area. I have a hard time thinking that it is the tranny because it shakes whether it is in park, 1, drive, over drive, or neutral in this rpm range, but I wouldn't completely rule it out.

I did a search for related items, but although I did find similar topics I did not read of any definate solutions. Any thoughts or solutions would be appreciated.

I inspected the cap and the flywheel area, and there doesn't seem to be anything worth noting.

Is there any way that vacuum lines would do this?

I was looking for possible loose or disconnected vacuum lines and I noticed a line that was broke off of the exhaust after the union where the two sides of the exhaust come together out of the engine. I loosely reattached the line by hand and the chirping sound seemed to go away. I took it for a drive and the shaking was still there, but it seemed to be reduced. Due to rust and such the line isn't on very tight, and I'm sure is still leaking. Is it possible that I found the problem here? Any thoughts?

Is it possible that this is the harmonic balancer going bad?