Clonk in the transfer case
 

94 Ford F-150 351 4x4. I have had the truck for a month or so and I keep feeling a clunk when it downshifts or if I let off the gas real quick. U joints are new. I believe it’s coming out of the transfer case.if I twist the drive shaft I can hear a clonking/chain rattle sound coming from the output shaft on the transfer case..the bushing and seal look new as well.the yoke has a tiny bit of up and down slack.i got a new bushing hoping that was it and the slack is the same as new bushing. Please don’t say the chain needs to be replaced lol.it has 150k on the truck.

You'd have seen the chain's condition when you disassembled the t-case to replace the bushing. Right?

No I just pulled the t case output tail housing.i don’t recall seeing it.

In any case, the chain isn't used in 2WD. Any slop is in the mainshaft stack. If it had worn to be sloppy enough to feel, the whole t-case would be worn out, so that slop is designed in.

But the clunk you feel is more likely coming from broken engine/trans mounts. Click these & read the captions:

https://www.supermotors.net/getfile/...l/04engmtr.jpg
(phone app link)

https://www.supermotors.net/getfile/...009_151851.jpg
(phone app link)

https://www.supermotors.net/getfile/...ginemounts.jpg
(phone app link)

I feel a little better now knowing that but I got my gf to push e brake and hit the gas a little and I did not see anything Moving a lot. My mounts do not looked cracked. Should I hit the brakes and hit the has even harder to see if it will jump up?

I found a good bit of movement when in drive and holding the brakes while hitting the gas hard. My other cars motors don’t move at all.i ordered both mounts.praying that’s it.

I'd try to cancel the order, and get these:

https://www.supermotors.net/getfile/...016_202943.jpg
(phone app link)

They're substantially more-expensive, but they'll never break. Original-style ones always break - even MotorCraft mounts.

Update I replaced the motor mounts and I drove it down the road and it went away a little but it’s still their.it will only do it when I get to 50 and above and let off real fast or hit the gas fast. I did not replace the tranny mount.i got a big crow bar and pried up it moved maybe half a inch and that was it.i thought it was good. Anything else?

Hard to say without seeing all the truck's details in your signature as this caption explains:

https://www.supermotors.net/getfile/...oorsticker.jpg
(phone app link)

Get an assistant you trust to sit in the truck holding the brake, shifting between 1st & Rev., revving the engine slightly while you crawl around underneath (all tires chocked fore&aft) listening for the clunk.

https://www.supermotors.net/getfile/...tethoscope.jpg
(phone app link)