So the angle of my rear axle may be the trouble?
Sorry, didn't see this until just now.
It's always possible. Lots of us need tapered shims after a lift to correct the now-changed pinion angle. And lots of new longer driveshafts have been purchased because of an incorrect pinion angle skewing the measurement results.
The more the pinion points downward, the longer the shaft needs to be to reach.
But as we like to say here "Every Bronco is Different" so while I've never personally been involved with a Bronco that needed a longer rear driveshaft after a suspension lift of up to and including 4" of lift, that does not mean that it can't happen.
You would need to check yours because there are so many possible variations. Including the incorrectly angled one from the factory (or dealer) on my friend's '76 Ranger. Bought brand new, has left it bone stock and has had to replace rear u-joints every 7 to 10 thousand miles for the last 42 years.
The last two years are left out because we fixed the "u-joint problem" with a pair of tapered shims!
So check yours with the full weight of the rig on the ground. Rear pinion should be pointed almost straight up into the centerline of the driveshaft. Most books call for straight into the shaft, but best results for Broncos are actually 1 to 2 degrees below the centerline.
Let us know what you find out with yours.
Paul