So evidently I can't even get a Lego Rover to work correctly.
I've got to the point where the three parts are connected into a rolling chassis. And I stopped there to make sure everything is correct, but there is a problem.
Neutral, everything is as it should be.
In R, diff in L or H, it rolls fine... either direction.
In D, L or H, it chatters a bit, then builds up a lot of tension, enough that the wheels go spinning when you lift it up.
When it is in full neutral in the t-case and in the gearbox, it rolls fine, and all the gearing works, bypassing the t-case and transmission as it should.
The same problem is evident if you put drive to the transmission, or the engine, or the wheels.
Actually, I think I figured it out. I think the u-joints on the longer shaft coming from the engine to the forward gears were binding. It seemed to be the thing that made the difference when I removed it. And then I found a video about the issue on YouTube. But the fix from that didn't work. My u-joints were installed properly. I think the design is just at the limits of what is possible for those, so my build of the whole chassis seems to have put it past the limits. Anyway, I removed the shaft, and rerouted the power from the engine to the transfer case instead of through that shaft to the transmission. This seems to have done the job.
But now that the binding is gone I can hear how clacky the top end of that Rover engine is......