1998 Jeep Grand cherokee driveline problem

I have a 1998 jeep grand cherokee, 5.2 with quadra trac.

When I make a tight turn at low speed, especially while backing up, the car starts laboring like something is holding it back. I have to give it a lot more gas to get it to go. Someone suggested changing the differential fluid and putting in a limited slip additive. I did that, got the additive from the chrysler dealer, and used 80-90 weight. It improved some but did not cure the problem. I have also changed the fluid in the transfer case. Used Dexron/Mercon automatic trans fluid.

Have not changed the front differential fluid yet. It is not positraction in the front.