My truck has always had the clunk when shifting to drive or reverse but today I noticed that if I have the shifter in 1st idling across a parking lot and tough the gas it makes a similar thump like it clicked into gear and when I let off the gas still moving slowly it thumps again and the truck slows down....

Also noticed that in drive going town a pretty long fairly steep hill if I let off the gas the motor slows the truck down.... Even going down the hill....

I replaced the rear ujoints, opened cleaned and refilled the rear diff (old oil smelled burnt) about 2 months ago.

I'm not sure how long it had been doing this but I just noticed it now.

I'm also getting a vibration on the hwy but I think that could be because I have the front hubs locked (t-case in 2wd) since it is winter and all...

I opened the hood and reved the motor a few times while looking at the motor mounds and they don't seem to be bad...

Any ideas?

 

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I would take a look at that rear end. You said the old oil smelled burnt, so there is a good indicator of where to start.

Just my .02

 

When I changed the fluid I cleaned it all out good... Everything looked ok. Nothing broken or cracked anyways...
Any ideas what to look for?

 

Any discoloration from getting hot, thus causing warping. Any uneven wear patterns, broken teeth, etc.

 

Didn't notice any discoloration or broken/chipped teeth.

I swapped my leaf springs the other week and noticed a "vibration" when going slow that I never noticed before as if a ujoint was binding but they are both new on the back end

 

Did you change the pinion angle when you swapped leaf springs? Did you replace them with some from another f150, or did you use some stiffer springs to give some lift?

 

Well if the rear end looked good maybe the u-joints or if you ahve a midship carrier bearing maybe there. Check the rear wheels for oil leaks at the hub sleeved axle ends. Maybe you have one going out.

When my carrier bearinf started to go I could hear a hum at around 40 mph

 

Oh I did not notice that. Good call

 

It had 4 leaf springs in it.
I used 5 leaf from a bronco.


I gained about 3/4". The Taller springs were the original

 

If I Left the front of the spring on the table and picked the back end up so the centers were even then the middle pieces lines up if that makes sense

 

I wouldn't think that 3/4" would cause an issue.. Hmmm... I would look over that rear-end with a fine-tooth comb, mainly because you said that the oil smelled burnt, and now you have clunking issues that sound rear-end related to me. Also check the other areas that DRRXR has said, he offered some great advice.

 

No carrier bearing on this truck.
The rear ujoints are both new(I still have to do the front ones hopefully next weekend)
I had the rear wheels off when I did the springs and didn't notice any wet spots.

 

Just because something looked good several weeks ago, doesn't automatically mean that everything is good now... Start looking..

 

Good point. When I bring it to work to do my lift I will open the diff up.

It has always had a hard shift/ clunk going into drive or reverse when parked. I'm not sure how much is normal tho

I've only put maybe 5000km on the truck in the last year and a half