old but not rusty

Purchased my F150 in February, 2012. 58 k actual miles, looks good, runs and rides good, transmission has been a pain. Shifted ok, but lots of noise and slack. Had a rear end installed, but still had noise. Had transmission rebuilt, but still had noise. Noise and slack were actually worse than before rebuild. Noise begins in first, fades out, then comes back louder at 40/45 mph. Slack when you let off and then accelerate. Folks who rebuilt transmission said 1990 AOD does this. After much discussion they actually installed a second rebuilt tranny, it was better, but no better than it was before 1st rebuild. Is there a later model transmission that will work in this truck. I am spending lots of money to try to have a really nice truck. I am tired of dealing w/transmissions. Looking for suggestions on possible repair or newer model auto/trans that will bolt in.