A good additive is made by Lucas. They make a product to stop slipping. You can get it at any auto parts store. Years ago my friend had an old car that we were building for a demo car. The trans was slipping really bad. We added 2 quarts of the lucas stuff and we were able to do burnouts with it. Just recently a coworker was having some slipping issues with his old mustang (one of the crappy 4cyl ones.) I told him to try lucas, and he did and said that it started working within a few miles. Try it as a last ditch effort, but eventually it's gonna need a rebuild. Cost is probably anywhere from 1,200 to 2,000.

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2003 Ranger FX4 level II. 3" BL, 33x12.50 GT Radial Mud Claws, Custom front and rear bumpers with winch hookups, custom flatbed, with hi-lift, and full size spare mount, 12,000 lb winch mounted under bed, Electric fan, and of course a flowmaster.