Rear end clunking? Shifting gears?

I have noticed once in awhile my rear end seems to clunk and hop, I hear a thud and it sounds like you ran over a curb. This is a Ford L9000, single axle 23k rear end with a 9 speed. The slip joint on the drive shaft has been greased but no grease has come out of the splines yet, is this normal? I've pumped it full of grease till no more would be accepted. 774k on the truck. The rear end has an air valve on the dash to lock it, but I've never used it while try to drive forward. Clunking noises are only heard in the low range or after a turn it seems. I don't hear any whining noises or anything else from the rear end.

I also posted a video of me shifting through the 9 speed. I've pretty much gotten it down, I just need to remember to be smooth and wait for the RPM's to drop before up shifting. I some times double clutch depending on how I accelerate and what gears I'm going to, as the shifter is a bit sloppy. Critique welcome, some times I run the max RPM to 1,700 and let off the throttle, wait a second to pull out then wait a second or two for the RPM's to drop before dropping in the next gear to avoid grinding.

[ame]http://www.youtube.com/watch?v=U98--jOEEjc[/ame]

 

The drive shaft is solid, I've tried to shake it and it wont budge at all. On the slip joint on the steering shaft, after greasing it then driving it, grease has started to come out around the splines, I thought this was normal. I thought it was normal for old grease to purge so new grease could be introduced.

I'm still trying to figure out why it can't move, should I remove the zerk fitting and let pressure fling grease out? I greased it over a two week period, grease, drive, grease then drive. The only reason I greased it a second time was because there was no grease coming out the spline. The truck had been sitting awhile, I figured this was the case. The truck had not been greased in I assume 5 years. I greased the ujoints until grease started coming out, it was alot of pumping.

 

Thanks for the info.

 

I like that spring and shackle theory. There are times after just turning a corner and I'm double clutching and not in gear yet and it's done it. Remember, it feels like you hit a bump, so its possible something is shifting down and it probably is the springs. The rear end never gets hot, no whining noise, no vibration or binding, I'm just paranoid.

 

Hmm, I don't think I have a torque arm on this truck. I've only seen that with air ride. Torque arm is what is on the differential to frame, correct?