Get yourself a helper and have them shift into 2 and 3 while you watch the levers on the steering column by the exhaust manifold. One of the levers should move up and down while shifting, then have them shift between R and 1, the other lever should move up and down. If it moves, then look underneath where the rods attach to the levers on the trans and go through the same procedure. If you see movement there, the problem is inside the tranny, possibly a broken shifter fork.