I'm posting here a problem I have that I wrote about on the CJ3B Forum before Tapatalk locked most of us out.  Here's a link to the thread:

https://www.tapatalk.com/groups/cj3b_bulletin_board/backing-up-or-not-t6329.html

For those of you that read it over on the 3B Forum, I now have an update.  Those just here on the 3A Forum, I'll summarize before the update.

I thought this problem was my rear axle. I tore into it to add shims and have the ring/pinion checked.  After the first shop tore everything out, and told me I needed a new one, I found another guy who rebuilds axles for a living.  During the teardown, I found some undiscovered damage so the axle needed rebuilding anyways.  It's a Dana 41 that a PO had swapped in long before I got it and as long as it works, I'll leave it.

After reinstalling it, I did another test drive.  The jeep still locked up in reverse when the engine was disengaged.  So I was wrong about the axle causing problems.  Now I move to the gear boxes.

Update:  With the jeep on stands, I remove the shift tower and observe.  I slowly let the clutch out to see the gears spinning and note what happens when I push the clutch peddle back in to disengage the engine.  Note:  Each shift is done with the tower and then removed to observe the transmission gears.

First gear 2WD - smooth stop on rotation (I didn't check 2nd or 3rd due to higher rpms and my issue is with reverse).

First gear 4WD/ hi and low - smooth stop on rotation

Reverse 2WD - sudden stop with a definite "clunk" and what may appear as a back lash as the gears bounce back from interference.

Reverse 4WD hi range - same sudden stop/clunk with the gear bounce.

Reverse 4WD low range - appeared to be steady stop without the clunk or bounce.

I also checked this with the Transfer case in Neutral.  With the transmission in 1st gear and then in Reverse.  In either case no clunk or bounce of the gears.  Just a smooth rapid deceleration to stop.

After wiping the gear oil off my foot and floor pan, I repeated the above with the tower in place.  This allowed me to let the clutch peddle all the way out while double checking what I did above where I was just "feathering" the peddle in an attempt to control the spinning gears ( and oil sling).  In each test, I got the same results. 

It appears I have interference in the transfer case in hi range but only when I'm in reverse.  I feel like there's a remove and inspect coming but are there any thoughts as to what may be happening?

I agree with 1955CJ-5.  The High/Low slider gear in the TC might be catching on something when in the 2WD position reverse.  When in 2W forward the gear action pushes the obstruction away.  In reverse, the obstruction slides into the gear and locks the gear against the case.

I would also pull the TC pan and see if something falls out.

Ok I've managed to spend a few minutes with my jeep and have an update. 

In my original post on the 3B Forum, Oldtime asked if I'd removed the drive shaft yet.  That was probably due back then to me thinking it was the axle.

Since the test above (freshly rebuilt rear axle), I thought it would be great to see the transfer gears (like I was able to see the transmission gears.  After draining (noted above), with pan off and gears exposed, I shifted to spinning one tire (other was blocked) to see what they were doing.  Nothing looked out of place and spinning the tire gave me nothing.  Everything behaved normally.  I wondered if removing all the rear axle weight would make it easier, so I disconnected the driveshaft and repeated the testing.

So, with the driveshaft now disconnected and the gears visible, I'd been spinning by hand the hand brake drum and shifting in and out (I actually pinned the clutch peddle to the floor to disengage the engine for this) to see if I could see something.  Nope.

Knowing that there was residual lube on all the bearings, today I used the engine to spin the gears (my thinking was maybe a video of what's going on under the jeep).  Drive shaft still disconnected, I ran 1st gear and pushed the clutch - normal stop on the gears (by this I mean once power was removed the gears would slow and stop in a smooth fashion without the hard jerk noted above).  Reverse showed the same.  Now I'm confused.  Then I reconnected the driveshaft and the test returned to what it was before.

Reverse 2WD clutch in - jamb stop.  Same in 4WD hi.  Normal stop in 4WD low.

So now, I'm wondering what changes when I connect the rear flange to the driveshaft that would cause this.  The output shaft nut stayed torqued.  The flange bolts aren't hitting any of the hand brake components.  Why only in high and not low? 

It's probably something simple but I'm not seeing it yet.

I think the idea of getting a video is worthwhile. Something must be moving/sliding depending on the direction of rotation or power application. The intermediate gear has those angled(helical?) teeth that may push the meshing gear one way or the other.

Is there someone who can help you?  Maybe even a polished piece of metal and you might see the reflection of the gears from above? Does the 3b have the large floor panel that exposes the transmission/transfer case area, allowing you to look down?

Another unique problem!

Rus, it was just a "SWAG" as it all seemed to work until you hooked up the shaft, as I understood your message.  It made me think of "angles" and "leverage" that might be fighting the system.  I've forgotten all you've changed, and tried so far.  Good Luck watching it operate backwards in a mirror.

Is the front drive shaft connected?

Could you drive it in forward and reverse with the rear driveshaft off while in 4WD, i.e. front wheel drive?

My experience with having a drive shaft out of phase/yoke alignment is that they have a low rumbling vibration at 25-35 mph. Not really noticeable at slow speed.

Since you said "in reverse pushing the clutch", could it be play in the main shaft that when the clutch pressure is off of the spline, it  allows it to move forward and bind when rotating in reverse?  Without the driveshaft installed, I'm thinkng the play can be absorbed, but the resistance of the driveline pushes it forward, maybe???  That would be really hard to duplicate and "watch what is happening inside", except maybe with wheels off the ground so you're not "driving", but then the "freewheeling" may not be enough resistance for it to bind.  Maybe add a little E-brake to simulate "load".  Just guessing out loud what all I would try.