Have an 87 Jimmy 4x4 that starting slipping yesterday while driving. Would get up to about 20 mph and it would just idle and only move cause it was coasting. I could shift down to 2nd on the automatic and it would run but of course could not do that long. I checked the fluid and it needed some. Put it in and it still does it. Any thoughts. Any bottle wonder to pour in that will fix it or is it needing rebuilt?