I have an 87 F150 4 x 4 with 320 and AOD tranny. All I can say is my truck clunks. When I'm driving and get on it the truck clunks. Its a clunking sound coming from underneath. I can feel it and hear it. At first I thought maybe it was the U joints but they are new. It kind of feels like its coming from the front of the truck but I can't be certain. When i drive it normal it doesn't make the sound. I test drove an SUV once that had the same problem but the sound was coming from the rear of the SUV. I figured the rear was bad so I didn't buy it. Any ideas?

 

Check your motor mounts and trans mount.

 

Ok I checked them out and they are fine. This only seems to happen when I press the pedal over half way down while I'm driving. I'm wondering if it could be a fuel problem? It doesn't feel like its bucking because I know what that feels like. Its just a clunking sound that I can feel too. If its the U Joints on the driveshaft it would do that all the time right? I'm wondering........this truck has a 2 inch suspension lift. Would that make the drive shaft too short when its lifted 2 inches? I looked at the U joints and they look new. I've only had this truck since January. It seems like the person before me might have thought it was the U joints too and replaced them. But then since it didn't fix the problem he just sold it. I test drove a truck once that had an electric brake control for a trailer installed under the dash. I test drove it nice at first and it drove fine. When I turned around to come back I stomped the pedal to see what it had and it clunked just like my truck does. I assumed it had a rear problem and with the brake control installed it was obvious to me that it was beat. So I didn't buy it. Just to clarify this isn't just 1 clunk. Its a steady clunking that stops when the rpms go down when it shifts. Then when it gets above 2k it starts clunking again.

 

With the trans in neutral and the parking brake applied get under and shake the rear drive shaft from side to side and up and down. Look for movement at either end. Now rotate the drive shaft clockwise and counter clockwise and look for any movement in the u-joints.
Could also be transmission or transfer case related.
If you think it could be an engine problem might be worth checking for any stored trouble codes.
Hopefully you will get other opinions.