Rear suspension

I think there are some threads about that? Maybe also try searching 

i can't help you with the rear arms but i wonder if the PMT Fab strut rods are compatible with the Little Shop spherical bushings?

https://pmtfabrication.com/product/1965-1974-ford-galaxie-strut-rod-kit

https://www.littleshopmfg.com/strut-rod-isolators-for-ford-cars/

So the tool I bought is pretty cheap, so it's hard to get the exact pinion angle, but as best as I can figure out, the pinion is leaning back by about 2 degrees more than the tailshaft is. So unless I could put a few washers between the transmission crossmember and the frame to get the tailshaft leaning back a degree or so more, I guess Spohn arms it is.

Engine and transmission mounts are rubber. Had this noise issue before replacing them as well. Front suspension also has all new rubber bushings installed in 2023. The poly bushings were installed in the rear in 2023, and I went poly due to issues getting rubber for some of the needed bushings.

I wonder if whoever setup the rear gear verified the index marks of the gears(if applicable) since a 3.50 gear set would be considered non-hunting, as they could be "timed". 9 Inch Gear Timing | Ford Muscle Cars Tech Forum

NVH -

It's a 2V 351C but with an aluminum four-barrel intake, likely an Edelbrock 2750 knockoff, so not huge power. A prior owner swapped the transmission for a C6.

Your noise could be caused by a host of things. The poly bushings are why you're hearing it. It's odd that the noise was made before and after a gear swap, so maybe disregard checking the gear set. 2 degrees is hardly anything, and with the poly bushings I don't imagine you're getting any flex difference between accel/decel. You can adjust the u-joint angles if you want, there might be enough play in the bolt holes on the control arms to get 2 degrees of movement. Otherwise upgrade to the spohn arms for the adjustability.

You keep saying 3k-3400 rpm, does it do it in 2nd gear at those rpms? Or is it more vehicle speed related?

Maybe your good wheel bearing isn't so good?

I worked on a 09 mustang once with poly rear bushings, and the tires on that were what caused the hum. When they say poly bushings transmit everything with nvh, they truly do transmit everything.

I bought a set of chassis ears years ago, I don't use them often, but for things like this they're invaluable when tracing down driving noises. They might be worth trying out here. I see you can buy them on ebay pretty cheap, cheaper then spohn arms are anyways.

So I went ahead and ordered the Spohn arms. Got the uppers too, since reading up on experiences Foxbody owners had with theirs, it sounds like there are fewer problems when not mixing-and-matching components. I should get them in 1-8 days (they're in Pennsylvania and I'm in California, so probably closer to 8 days). I should be able to get them all installed in one night.

Supposedly poly bushings can be greased to fix squeaking and nvh, it's why some are impregnated with graphite.

I have seen those pinion yokes with dampers, most commonly on aam axles in one ton pickups. Never seen one on a 9 inch. Maybe have the driveshaft balanced? Just spit-balling here.

By "leaning back," I meant the axle leaning back, which pushes the pinion input up. So if you draw a line through the centerline of the pinion, the line would slope down from the front of the car to the back, so someone surfing on top of that line would be leaning back by a few degrees. So it's parallel to the transmission output shaft within a degree or two.