I have a 2014 F150 Limited and a few months ago I put the bilstien 5100s in at max height and removed the factory rear blocks and installed 3" blocks. I also put 20x9s with 35x11.50 tires on.

I live in Minnesota and like to use the 4 Auto feature in the winter, I used it a lot last winter with zero issues. We just got some bad weather and I put it in 4A for the first time after the above mods. When driving if I let off the gas then hit the gas again I get a very loud clunk/slam that is very unsettling. I heard it twice in a row and immediately put it back in 2wd and haven't tried anything else since.

Any ideas what this could possibly be? I'd really like to be able to use the 4A option as it's one if the reasons I purchased the truck...

 

I would look at greasing the rear drive shaft before I would do anything else, well of course the obvious look for loose stuff

 

Maybe the angle of the driveshaft is just enough to give it that little bit of play.
So yeah, grease is an idea.

 

How about the front drive shaft? It doesn't make any noise in 2wd only 4...

When greasing do you remove the drive shaft and grease the ends?

 

 

Thanks god for his videos

 

I haven't had a chance to try it yet since it's been about -20 degrees here the last week. Once it warms up I'll give it a shot. I still think there's another issue though as it only does it in 4wd and works perfect in 2wd....

 

Well, at least you can rule out the rear driveshaft...
I'm sticking to the angle giving it that little extra play in the front shaft.