94 GMC 4x4 not shifting?

i have a lifted 94 gmc with the 4l60e. we have had two transmissions in it already. the first one had non reverse or 3 and 4 gear. the second has reverse but still no 3 or 4. when you put it in drive it starts in 2 gear but when you get up to around 20, i can physically put it in 1st and you can feel it down shift. what is causing this problem. bad trans, sensors, faulty plugs. please help!!!