Tried this before to no avail, maybe if I explain it better, I'll come up with a solution.

Relatively stock '87 4Runner, 22RE, W56 tranny, ADD front end. Recently had a rear u-joint fail, dropping the driveshaft at 60MPH. Replaced both front yokes on rear shaft, but while running around for parts, I used front wheel drive - for a total of maybe 45 miles.

Since replacing the rear shaft and resuming "normal" operation, the truck clunks when reversing. It will continue when I start forward again and usually goes away with a few rotations. The sound seems to be coming from the right front side of the drivetrain and is directly tied to the wheel rotation.

My first guess is the ADD system, perhaps it's engaging or partially engaged causing the sound. Anyone have any suggestions or ideas on what to check, It's beginning to drive me nuts...

make sure your driveshaft is phased correctly

How do I check the driveshaft is in phase?

Why would it only happen when reversing?

you have to make sure you drive shaft is in the correct spline, and not off a tooth
4crawler has a good webpage about drive line troubles.

Someone correct me if I am wrong...
But I did not know that an out of phase d-shaft would have any affect at slow speed.

I have heard that it will be very noticeable at low speed, easy to check, just make sure the u-joint bearing caps are lined up end to end:

http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml#Phasing

I don't think it'll make a clunking noise, though. Usually more of the rough, uneven, pulsing feeling as it turns. Might be worth checking the slip yoke splines for damage, they may have been affected by the u-joint failure.

Very or bearly noticeable? <img src="/forums/images/graemlins/notooth.gif" alt="" />

Turns out the driveshaft is indeed out of sync. I'll get to it tomorrow morning and report back if it works or not.

Thanks for all the help!\

Did ya fix it?

Afraid not, phased the driveshaft correctly, problem's still there. I'll get a buddy to help out this weekend with an extra pair of eyes and ears to see if we can isolate the source.