88 1500 clunking in driveline

I have an 88 stepside and it's been clunking a little worse every day, not too noticeable, but my friend brought it up the other day it felt worse than before, it will only do it after coming to a normal stop from anything over like 40 mph, then when I let off the brake, it feels like the whole rear of the truck shifts and drops, then you hear the clunk, it does nothing when I rock it back and forth in Park, and I just replaced the U-joints on the driveshaft, think it could be a front transmission bearing?, oh yeah that's another thing, the tranny shifts HARD into 3rd, no other gears, just 3rd, like it has a shift kit, any Ideas?
thanks

The modulator on the transmmisson could be bad or isn't getting any vaccum from the intake port. Repair or Replace is the cheaper solution...

could absolutley not be the modulator on the transmission. The th700r4 is electronicly controlled there is not a vacuum modulator. I don't know what the fix is, but my 88 has done it forever, with all three trannies.

Could be the slip yoke is worn and is binding. As you come to a stop the rear rises and the slip yoke extends a little, then binds instead of slipping back in. When you leave from a stop it overcomes the bind and you get a 'clunk'.

This is more common in lifted trucks.

Another possibility is bad tranny mounts.

Rene

Yep have to agree with broughy84 about the modulator, its electronically controlled if you have the original setup, TFI. my 89 silverado isn't TFI, nor has it the TH400R trans. Another possible solution could be as tRustyK5 suggests. Mebie even loosen flywheel bolts (like mine had) or a bad torque converter. If its very serious, a quick stop at a transmission shop could be in order...

Ooooohhh....I didn't think about loose tourqe conveter bolts......that's a possibility.

Ooooohhh....I didn't think about loose tourqe conveter bolts......that's a possibility.

loose torque converter bolts?, I don't think that's it, it just feels like the whole truck drops in the rear end when I let off the brakes, and you can feel a thud, sometimes it doesn't do it until I step on the gas or roll a few feet, think it could be a bearing in the transfer case? and any other ideas about the slamming into 3rd gear? only 3rd, 1-2 is fine and 3-4 is fine, but it slams into 3rd.

Have you looked really good at your springs to see if you have a broken/cracked leaf spring? That would only be a reason for the rear to drop down when you let up on the brakes, the motion should propel you going forwarded...just a guess, but wouldn't hurt to take a closer look.

My friend had a '91 with the same problem. I would suggest inspecting all the leafspring mounts because it sounds like the rearend is twisting(rolling) axialy. Loose torque converter bolts would be normally heard or felt at idle, and when the engine is being started or shut off. As for the hard shifting, i would suspect an accumulator inside the transmission has failed. These are placed in between the valve controling the clutchpack, and the clutchpack itself. While the fluid is being applied to the clutch, it's also acting on the accumulator, which is a piston and spring assembly. While the fluid is pressing on the acc. piston, it slows the engagment on the clutchpack.

it may be your transmission mount