Drivetrain problem...need help please!

[waianae_supra]

I have a 94 Bronco with 161k on it. All of a sudden, I'm having
massive drivetrain problems that make it undriveable.

On the 21st of Nov, I took it offroad for a 20 mile drive into the
mountains. Had ZERO problems with 2 or 4 wheel drive.

I then parked it for about 2 weeks, and then used it for a run
to the local dump. It's 6 miles to the dump, and the truck
drove fine on the way there, in 2 wheel drive. To toss out
the junk I brought, I did a tight 180 to back up to the
dumpster. I put the truck in reverse and started backing
up. That's when the "clunking" started. It felt as though
the rear brakes were grabbing quickly and then letting go.
Because I was already at the dump, I backed it up to
where I needed to be, slowly, encountering a couple more
clunks in the process.

I shifted to park, offloaded, and then hopped in to get off the
property, in case it broke down completely.

In drive, I encountered more clunks, this time seeming to
come from the front end. I got off the property and took
a quick look under the truck. I couldn't see anything
obvious, and needing to get it someplace safe, I continued
to drive it home (you NEVER leave a nice vehicle parked on
the side of the road on the leeward coast in Hawaii). On
the 6 mile drive back home, the clunks were sporadic and
light, seeming to mostly come from the front end. During
the drive, I shifted to 4 wheel drive, thinking the TC was
partially engaging somehow. I limped it home at no more
than 25mph.

Thinking the front hubs got screwed up somehow, I tore apart
the left front hub, but didn't see anything amiss. I re-assembled it
and then decided to look further into the drive train.

In my driveway, I shifted from P to R and then D, encountering
massive clunks on each shift. I could feel the initial "load" on
the tranny, as is normal. About 1 second later, there's a massive
lurch to the drivetrain and a very loud clunk/thunk. A neighbor
3 houses down could hear it.

I had my wife shift the tranny (P/R/D) while I was scoping out
the drive shafts. On each shift, the driveshaft loads up like
normal, and then spins almost 1 revolution, which produces
the "thunks". Both front and rear driveshafts do this. This also
makes the truck lurch quite heavily.

This happens with the truck in 2 or 4 drive. I can shift from
P to N without any problems, and the trucks rolls down my
driveway just fine with no clunks. As soon as I put it in R
or D, the thunks are back, even when just moving in the
driveway. If I shift from P to R or D, wait out the thunks,
and then attempt to drive it, I can feel either the front or
rear wheels "catch" and then disengage. It's frustrating
because it's either the front or rear, never just one of them
consistently.

Because the problem seems to happen front and rear, that
rules out the diffs and front hubs. That leaves the TC and
tranny and perhaps the electronics (auto hubs). Which seems
the best bet? I don't have a lot of money to sink into the
truck, with a new baby and all, so I have to be prudent in
my choice of repairs. Which would be the best candidate?
My thoughts are the TC, but I'd like some hopefully informed
opinions.

Sorry for being long-winded...just wanted to explain this as
much as possible.

TIA!

WS

[Truck]

If you've observed the driveshafts acting abnormally, you should probably start with the TC. (Given that the problem isn't in the U-joints...or CV joints.) Check and make sure both shafts are securely attached to the TC. Then start on the TC. I take it the shift pattern was unnaffected. (that is, this is an AT which still shifts when it's supposed to.)

[MagicRat]

My jeep does the same thing. Its your TC.
Have a look at the shift linkages that hold the TC in high or low range, and those that hold it in 2 or 4wd. They may not be actuating the shift linkage all the way, so the TC is slipping in and out of "neutral" (the TC neutral, not the transmission neutral. )

[waianae_supra]

Thanks for the replies. I'm suspecting the TC, but I'd like to be more
than 50% sure before I start ripping into it or looking for a replacement.

As far as I can tell, all CVs and UJs are tight. By hand, I can't feel
any slop in the joints.

As for shifting, the tranny shifts fine, although I never got above
25 or so. I may have hit 30-35 when I had no choice, but everything
tranny-related seemed to work ok.

As for the linkage on the TC, is that internal? Seeing it's a push-
button 4 wheel drive system, I'm guessing internal?

Also, could an electronic signal or lack of one cause this problem?

When I select 4x4 on the dash, I can hear all the relays kick on
like normal. I did a cursory check on the wiring and didn't see
anything amiss.

Also, as a perhaps a long-term temp fix, could I remove the TC,
both drive shafts, and replace the rear drive driveshaft with one
from a 2 wheel drive F150?

Again, thanks in advance.

WS