My 1998 4WD (200k mile) Tahoe transmission stopped shifting yesterday while on the interstate. I was attempting to pass and the engine just revved. I managed to cruise in 1st and 2nd gears to a shop, where the fluid was checked (2 qts low) and filled. But test drive determined that it will not shift out of 2nd. Have not checked line that runs above skid plate yet. NO previous problems noted. No visible leaks on ground or signs of leaks on trans housing. . No codes in diagnostic computer. Any suggestions as to possible cause/cure as I want to go to trans shop informed rather than rely 100% on their advice/diagnosis. Many thanks in advance.