My 2004 Baja Turbo 5 speed was slipping in fourth gear and now in other gears.The RPMs would go to like six with loss of acceleration. When I check the differential it was at the low level I filled to the top level. Then after sitting for several days I checked it again and it was dry with a puddle of fluid on my pavement. I did have the viscous coupler replaced about five 6,000 miles ago. Any suggestions?