Doesn't want to stay in 3rd or 5th?

I think you answered your own question.

;-)

Ron

Rocky; Just went through the same situation. I found that one of my U-joint yokes was not tight enough and permitted one of the spline shaft ends to partially pull out. Take the metal shift gate and rectangular cover off the shifter, which will act as a stopper in 3rd & 5th - test to see if you can then engage the gears. 2nd and 4th will also engage prior to where they once did in the shift gate. If so, then one of the yokes has probably let the spline end slip. Also check your shift linkage center support for tightness. As r mcall states, you've answered your own question, a linkage adjustment is probably required.
I found that my adjustment required each spline to almost be "bottomed-out" in each yoke, then a final adjustment at the adjustment section of the shift rod.
You will need a second person's help as they will SLOWLY move through the gear selection as you watch each yoke for linkage rod movement.

Generally, it is a sign the trunion bearing is loose and flopping around. It is located on the driver side frame rail forward of the rear tire.

Thanks all-

While I was pretty sure it was linkage, I didn't have time to get to it yesterday, but have to attack it today, before our run.

Of course you always worry with the ZF that any issue is not a simple one, just due to the consequences of a failure.

Thanks for the ideas.

Rocky

Curious to see how this works out as I sometimes have a problem going into reverse.

So, quick update - I fixed the problem...

It was a slipping splined linkage on the rear-most part of the rear U-Joint.

I first checked the trunion nut, but that was tight.

Then I found that there are not 2 U joints, but three - there is one you can partially access by removing the ashtray...

Luckily we were able to see the unwanted movement of the shaft / spline pretty easily. I didn't want to take my console apart!

Rocky