Why would the rear diferential grind and jump when turning under heavy four wheel drive load, but run smooth under all other conditions? 1995 f250 power stroke.

Lack of proper friction modifier may be responsible for the problem. I would drain the rear differential, after getting it warm. This should be done every 100,000 miles. Add 8 oz. of Friction Modifier (XL-3, Blue) first. Then, add 7.5 pts. 75W-140 GL-5 Synthetic Gear Oil (Mobil 1 or Amsoil). Fill to 1/4" to 9/16" below the filler hole. Last weekend my neighbor overfilled his Ford's differential and it blew the left seal out.

If the situation continues, remove the differential cover and check everything out. Afterwards, see if you can install a Lube Locker Reusable Differential Cover Gasket, which are awesome.

Also, go over the complete drive train, paying particular attention to the condition of the universal joints or driveshaft bearings--they may need replacing and/or greasing. I recommend greasing with NLGI #2 synthetic grease (Mobil 1, Amsoil GLC, or Valvoline SynPower) every 20,000 miles.

Are you doing this on dry pavement? 4wd needs to slip when turning. If it is on dry pavement, take it out of 4wd.