2001 tahoe won't shift into 3rd or 4th

i have a 2001 chevy tahoe automatic 4 speed and the torque converter started to hunt for a gear when it downshifted on highway and then failed to shift into 3rd gear altogether. So I only have first and second gear. Could this be electronic? ie. solinoid for TCC? I only have 69,000 miles on the truck and never hauled anything. It's out of warranty of course. Any ideas where to start? I don't trust dealer. I checked tranny fluid and it was down a pint and leveled it off. Still have problem.
Rob

check you ignition switch - known problem on chevys with electronic controlled trannys