Thanks, guys. I had downloaded the manual the day I bought the mower, soaked the thing in PB Blaster last week (THE BEST RUST buster I have ever used, bar none). Rust, corrosion, or sticking linkage had nothing to do with this, however. The control arm on the transaxle was moving its full limit. Discovered that only after I took off the spring loaded nut (washers, spring, nut) and could see the travel in the control arm slot. I knew the problem was not linkage: I had disconnected the linkage in stages to see what was moving its full travel and what might not be, found no problems, and got down to the transaxle control arm. Loosening the bolt holding it to the shaft caused everything to travel as it should. I thought it might have been on a spline and slipped a notch, but I finally inched the bolt out that held it on and it's on a square end of a shaft, and I knew it had not slipped 90 degrees. Long story short, I still don't know what the probem was, , because the control arm is not traveling as far as it had originally in the slot (not to its limit), but the shift is perfect, forward, neutral, and reverse.
About PB Blaster. Last year I was trying to replace all the spndle bearings in a 3-spindle Bush Hog finish mower. I soaked them in a popular rust spray, and got to the point I was willing to sacrifice the shafts. I beat on them with a 16 lb sledge and they did not budge. Again and Again, day after day. Went on a forum and was told to get PB Blaster. I thought "this won't do any good," but as I've heard that my grandfather used to say, "It warn't no good like it was, Georgie." I sprayed PB Blaster on the shafts, and after a time went out and knocked all 3 out easily. Available at Wally World even. Try it.