sounds like a clunk noise when

[onecustom73]

i get off the gas going down the highway,its a 3/4 ton with 4.56 or 4.55,wich ever came factory,going down the highway and letin off the gas and get back on seems to make this kinda clunk noise around the tranny area,it would be my guess,you can feel it once you let off the gas,doesnt happin when your on the gas just when your going and  you drop off the gas it make that clunk noise that you can feel soo im lost,dont know what it is,i was thinkin maybe tranny mounts,but there not all cracked or anything,dont look bad,looked back at the transfer case mounts and there just frickin shot,both lowers,caint quite see the uppers,could this maybe be what givin a lilttle when i let off,help please!

[Lt.Del]

check pinion gear bearings on rear axle
check drive shaft u joints
shake the tail of the transmission very aggressivley while up under it, ensure it isn't mounts.
could be engine mounts as well

[DnStClr]

Yeah. It sounds like a u-joint is shot.

[onecustom73]

Alright guys i got underneath there this morning and started to kinda check this n that to see if i could feel anything loose,and sure enough,when your following the rear drive shaft back up to the transfer case well before you get to the u-joints and the little grease insert The drivshaft gets real small and then goes to the yoke and on to the u-joint,WELL right where the drivshaft goes from big to small there is some play back an forth,i dont think its the u-joins,its seems to be right when it goes into that small shaft that i know lets the drivshaft go in and out,or alows it to flex a lil,i think,soo that seems to be where its comin from.So what should i do now?

[onecustom73]

so pretty much with this two-piece drivshaft,when it goes into the smaller tube there's play right there,a lil up and down,wich i think is makin that clunkin noise when you get on and off the gas,you got to be in it a lil,dont feel it if you just barley on the gas,so now being 35 some years olf maybe the ol girl has had her days,do i need a new one or..?

[DnStClr]

That's the slip joint, and it can come out of the spline in the drive shaft just enough to cause that clunk when it goes back in . The torque converter keeps pressure on it, so that's why it clunks when it re-engages.  Is there a lube point right there where you can grease it?

[onecustom73]

yeah there is up on the yoke,i'd say about 3 inch up from when the driveshaft goes into the slip yoke,when it goes smaller!never have greased it before,with it not greased could that maybe be the reason there is a lil play in it,or should there be a lil,it has one of the bigger driveshafts i have seen on the ol girl,seems to be a lil bigger then most i see!

[DnStClr]

There is a little play in them. When the truck goes around a corner or the frame flexes (up and down) the yoke needs to slip out of the drive shaft a bit. The shaft splines shouldn't disengage tho. Worn u-joint can still be what's going on.

[VileZambonie]

The slip yoke should not be sloppy.

Also check the crossmember and mounts.

check u-joints as suggested.

Check for excessive backlash