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.
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.