The 86x0 and 84x0 are the same tractor behind the engine if the same age.  A lot of the problems with the 86's can be traced to the fact that Deere did not make a 400 hp tractor....if you were a died in the wool green guy and wanted a big tractor you couldn't buy one, so you got an 86 and asked it to do more then it could. The 84's didn't suffer this as much, and they had a bigger margin of safety built in to begin with. 

The pressure lube vs not thing has to be the front axle.  I'm not so sure it's a big deal.  No other 4wd tractor of that age has a pressure lubed front (or rear, for  the most part) axle.  I think the biggest problem was that the front axle had it's own lube compartment and Deere guys were not conditioned to deal with that.  Steiger or Versatile had front axle, rear axle, LF final, RF final, RR final, LR final, and transmission all seperate and knew they had to take care of them all. It could be argued that the pressure lube changeover merely provided a way to get progressive damage from a front final drive into the rest of the drivetrain. 

The pinion pin coming loose and knocking the lube pipe off is not in any way confined to the 8630....it can and does happen on any Deere that has two a two pinion differential.  I'm not sure if the 3 and 4 pinion designs had some way to prevent this or not.   We've had it on a 7520 and a 4010...the 4010 had the pin break, the 7520 just sheared the bolt that holds the pin in off.  The 4010 busted the pin up in little pieces and didn't do any further damage, 7520 knocked the tube off and trashed the transmission, but there is a possiblity the progressive damage could have been prevented if we had been smart enough to park it the minute we heard the "chunk" (which turned ou to be the pin coming out) instead of moving it the rest of the way home.