92 b2600i 4X4 help changing rear diff oil seal
 

I've had a leak coming from the rear differential.Gear oil gets splattered all around the bottom of bed.I bought a new oil seal.When I took out companion flange I couldn't get to oil seal behind that.My Chiltons book is vague about this.It's not showing a piece inbetween.I don't know how to remove this piece.Do I need any special tools? Just to confirm I use 75w90 or 85w90? Any help would be appreciated.I need to get this fixed.Thanks!