She just won't move??!!
 

Okay, I'll explain the situation and the outcome.
I have a 91 350SD that I just pulled the 3.5 out of and put in a 3.0. Just got it all hooked back up on Friday. Fired right up without hesitation. Let it run a little added fluids as it started warming up. Got in put it in drive went forward, put it in Reverse went backwards...did this a couple times. Checked the transmission fluid while it was still running, a little low so I added some, maybe a tad above the full line. Got back in the car (it had never been turned off since I was driving back and for the and then added transmission fluid) put it in drive....nothing, revved up the RPM's, no movement. Put it in Reverse....nothing.
If I put it in Park the transmission goes in Park like it's supposed to, when I put it in Reverse or Drive there is no engagement and I can roll the car like it is in Neutral.
I did not replace the transmission or adjust the Bowden cable when I removed the old motor. But I don't suspect the Bowden cable as I drove it back and forth in the driveway with no problems and now nothing.
Any suggestions or ideas?

Are you sure your shift linkage is connected correctly?

The linkage never was disconnected. But I did check to make sure nothing came unhooked.
But, as I said, I had been going back and forth in the driveway without difficulty and then nothing. No pop or clunk.
I'm leaning more to a vacuum problem but if it is bad enough to not allow the transmission to shift would there not be other vacuum problems?

Torque converter bolts all properly installed? If you left them out by accident (I've done stupider things myself), there may have been enough "drive" by the lugs touching to move the car a little, then it finally started to slip too much.

Has to be something similar. It's definitely not a vacuum problem.

^^^ Agreed. Did you use the original torque converter? If not maybe the replacement one is different. You should pull the cooler line to see if trans fluid gushes out while running. Be careful, you'll make a big mess.

Mine did that, and it was a sign of impending doom
 

My '81 is still in the tranny shop being rebuilt. (they had to get a couple of parts from the MB dealer) What I found when mine wouldn't move is the filter was clogged. I went through a couple of cycles of cleaning the pickup screen (and changing the filter once), putting all new fluid in which would fix it for a week or so, but in my case the bands/clutches were disintegrating and clogging the filter with tiny bits of friction facing material.

I'm supposed to get mine back from the shop on Friday.

All the bolts are in but I did use the torque converter from the 3.0 motor. So it has the original transmission that was in the 91 350SD but the torque converter came out of the 87 300SDL with the motor.
I completely drained everything and put in a new filter and gasket.

Maybe the torque converter still isn't full of ATF? Maybe there was a big slug of air that went through the system, including in the valve body?

I did the same conversion but we had no problems like this...Bit of a heads up though, if that 3.0 was sitting for a while, don't be surprised if it drives EXTREMELY slowly at first. The performance will improve over time.

Are you sure the torque converter is seated in the front pump properly? Maybe the ears that drive the pump are not engaged? Plumb a test gage in and check line pressure. The car I had with the "no drive" symptoms had those little drive ears sheared off the torque converter, thus no pump pressure.

I am going out right now to see if I can make a mess in the driveway (not that you can tell) by unhooking the line to the cooler and see if I have pressure.
The motor is great. It came out of a perfectly running 300SDL. The one I put in my wife's 300SD had been sitting for a long time. It smoked and drives slowly up the hill. But she loves driving the speed limit and taking 20 minutes to get up to it. It's gradually getting better.
I'll let you know how the pressure line goes.

Well, the good news is I didn't make much of a mess in the driveway.
The bad news is, there was no mess because there was no pressure from the line going to the cooler.
Tell me I don't have to pull this entire motor out again.

You don't have to pull the motor out. You can pull the transmission.

Do you think I should use the other torque converter?
I used the one from the donor vehicle rather than the one that was on the receiving transmission.