2004 F150 SCab 1st Order Driveline Vibration?

My truck is approaching 50,000 miles, original tires, no tranny problems ever, had it dealer flushed last summer at about 36,000
miles. Build date was 7/2004, it's the new style.
I started experiencing some hard vibration on the highways during acceleration. Dropping the trans into neutral did not stop the vibration. It's an intermittent problem, does not always show up between 40 and 55 mph, and can continue on up into the 70s. I think it's the 35 hz vibration?
I checked the drive shaft, it's a one piece unit with staked u- joints. The worst of all possibilities I've read. Seems that if the joints are bad the entire shaft needs replacement?
There are aftermarket 2-piece units as well. Apparently Ford moved over to the two-piece shaft and regular locking d-rings for the u- joints shortly after my truck was built. The staked u-joints affected a lot of other models, too it seems.
Well, before I go throwing expensive parts at my truck, I figure I'd at least get all new tires and front rotors/pads since the originals are looking worn and ratty. It might help, but if not, is there a way to pinpoint the issue?
Would it help to lift the back of the truck on jack stands and "accelerate" to 45-50 mph to see if the vibration occurs? Does tell me it's a u-joint or drive shaft (result is no different in my case).
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