I have an automatic transmission on my 86 F150 Lariat, and I've noticed that when I'm driving, if I take my foot off of the gas, I can hear the transmission clunk. It happens sometimes when I put my foot back on the gas (going 25, 30mph usually), but mostly happens if I have to take my foot off of the gas pedal to slow down.

I just replaced the transmission mount a few days ago, which is causing me to feel the transmission shift a bit (something that I was told would happen for a bit until the new mounts are worn in), but it didn't alleviate the clunking noise..

Any ideas? I'm hoping to get it fixed as soon as I can figure out what it is.

 

Could be the splines are worn in the yoke.

 

i hope you already checked your u-joints - thats the first thing i would look at for such a noise. with the parking brake set and the tranny in neutral, feel for slack in each joint in the driveline. the axle will have a bit of slack, but the u-joints should have almost no slack.
in a few cases, a bad u-joint won't show up right away when you inspect it, but will if you have a buddy hold the brake, shift to drive, rev the engine, shift to reverse, rev the engine, and repeat, as you're under there looking at the joints. just make sure its a friend you trust with your life, and preferably also use tire chocks and/or chain the vehicle down so it can't move even if he screws up