99 Tracker, auto, 2.0, 2wd, 163K miles, near perfect car, until now. When I put the shifter into reverse, it feels like the emergency brake is on. Won't move. After I drive it for 30-45 minutes, put the trans into reverse and it goes. Sensor? 

Maybe the emergency brake *was* on and didn't release properly?  Did the engine rev freely when you tried to back, or did it only rev to 1500 or so RPMs like if when you try to drive forward while stainding on the brakes?

Also, I have to ask, you're sure there was nothing behind one of your wheels acting like a chock, right?

Only does it in reverse. No, the emergency brake isn't on and nothing is chocking the tires. Engine reving freely in reverse.

Problem happens only when cold, like something (sensor?) isn;t  opening until warmed up. You guys are the experts, thought maybe someone else had the same problem.

usually your symptoms means it needs a rebuilt on non electronic automatics, Reverse is usually the first to go and usually works when warm at first.  I am not familiar with electronic suzuki autos, so my experience may or may not be true with yours. 

Non electric automatic tranny? How do you tell which one you have?

I haven't worked on autos in years, but I believe Suzuki after 95 went to electronic controlled ATs.  I find it hard to believe that there is a electronic signal that can cause what you are describing, but I dont know for sure.  Your symptoms are a classic example of a tranny calling for help.  if you have it serviced, have them look at it and I bet they will be able to tell that it is on its last leg.  the service might buy you some time.