How hard is it to replace the seal on the rear differential. 2001 Ford Ranger 4x4

 

I assume you mean pinion seal not axle seals or gasket for cover

On a scale of 1 to 10, 1 is changing the oil, 10 being automatic transmission rebuild for the first time, lol

It is a 7

It is just wrench work to remove pinion and install new seal, you should also do axle seals at the same time since axles need to be pulled back, check the bearings.
To reinstall requires some shimming to get everything back to specs, loads of videos on doing this, i.e. changing ring and pinion gears(ratios) which is same tear down and rebuild
So just a bit of a learning curve
Trust your common sense, and BLOCK the front tires, lol

And yes you could change ratios or install Limited slip at this time, although most 4x4 Rangers have Limited slip in rear differential
Look at drivers door label, at the bottom is AXLE and a number below it, get that number then look here: http://www.therangerstation.com/tech...le_codes.shtml