Whenever diagnosing performance issues, the mechanical condition of the engine is often overlooked. Especially with older vehicles, this is where I like to start. If you have low compression, or a loose timing chain, it can cause all kinds of performance problems that no amount of parts will fix other than replacing the broken or worn ones.
Here is where I suggest you start for how to diagnose performance problems, BTW, there is a video on how to use a vacuum gauge to find exhaust restrictions.
http://www.ericthecarguy.com/faq/solving-automotive-performance-issues
As for the leaks: https://www.ericthecarguy.com/faq/finding-and-fixing-leaks/finding-and-fixing-leaks
With the pinion seal, it’s quite possible that the pinion bearing is worn. At high mileage, rebuilding the rear differential and replacing the axle bearings is not a bad idea.
You might also find this video series helpful.
Good hunting.