ok.......yesterday i decided to change my atf since i hadnt done it in a while...transmission was running great til after the change....changed the fluid and filter..didnt drain the converter or anyhingjust the contents of the pan..used a respectable wix filter and multi purpose dexmerc fluid....well left to test run it down the road it was fine under normal load...if i laid into it would slip...ok figuring it might be a lil low i went back to the shop checked the fluid again and come to find out it was bout a quart low...put that in and test drove it....got better through out the drive home...took a 180mile drive today and seemed good til i figured i mess with a lightening.....she downshifted and slipped again...but was like i was in neutral.....checked the fluid again and it seem like it should have been ok...the level was at the bottom of the hatch marks...havent done anything yet....local shop says it should be 5 quarts for the pan...put 5 in and still slippin....i aint no mechanic...but like to do my own works....done this before to this truck and didnt have a problem....any ideas!!??

thank you in advance!!...its appreciated

trevor