When I reverse and then go forward I get a loud clunk coming from the transmission hump. Like someone is taking a hammer to it. Same thing happens when I go forward then reverse but not as loud. Truck runs great. Its a 88 F250 4x4 5 speed with a 10.25 rear.

 

Update. No play in u joints or yoke on rear. I don't know what could be the problem

 

Did you remove the driveshaft to check the joints? Play can be hard to detect with it installed.

Check the trans mount. I have seen the nuts missing from the studs before; trans slid side to side substantially and did not sound good.

 

No I did a quick inspection. Left it in gear to check u joints then put it in neutral to check the rear yoke. I was leaning towards the slip yoke.....I will take the driveshaft off this weekend and check the u joints again

 

Another update....it does it when coming to a rolling stop and turning both ways

 

Like GoinBoarding said, check the transmission mount bolts. I'd also check the bolts holding the cross member to the frame. My mount bolts were loose & I was missing the upper bolt for the cross member on the drivers said. Correcting these issues got rid of a clunk I had (one of many clunks though).

 

Okay I will check them once I get home

 

Sticking slip yoke can cause a clunk on take off. In my experience, that's less of a "hammer" clunk and more like dead blow or soft clunk. Does this have a slip yoke in the transfer case or in the driveshaft?

 

If the slip yoke is in the driveshaft, that's gonna most likely be the clunk. On my brother's Ranger, we just took the rear yoke loose, cut one of the bands holding the boot over the joint and greased it with bearing grease. A big tie-wrap worked to replace the band we removed. Hard clunk was gone after that.

 

My loose bolts sounded like a "crack" when turning a corner with a little speed. I think it was cross member mostly because I could kind of feel it in the seat.

 

The driveshaft goes into the transfer case. Heading outside now to check the trans mount

 

Well the trans mount is bolted up fine, no missing bolts. Could be shot but I will have to wait till Saturday to find out. No leaks from transfer case or rear diff......any other ideas? If I let off the clutch very slowly it won't clunk.

 

When I had a sticky slip yoke, it wouldn't clunk if I slowly accelerated. Anything quick/normal and it would. This was an auto, so little different load but same idea. You can test this by pulling the driveshaft, loading the yoke splines with grease, and reinstalling. If that cures it, either grease it once in a while or put a new yoke & U-joint in and be done with it. I replaced the yoke 150k miles ago and it has never clunked since.

 

I'm ganna try doing that. When I let off slowly it doesn't do it either. I'll take a pic of the transmission mount as well

 

How do you test the trans mount? You know how in a manual when you let off too soon the vehicle will jerk back and forth, my truck did that and it popped every time. I'm leaning towards the trans mount now. Would that be more plausible now with this new symptom?