Check the shifter cable. You may have bumped it or moved it so it's not shifting right. Also, did you tighten the stuffing box with the shaft turning or stopped? You should tighten it with the shaft turning slowly until water only drips a drop every 10-30 seconds or so. Are you using a graphite-based packing or wax-based? The packing can seize to the shaft if you tighten the stuffing box with the shaft stopped and then leave it for any period of time. :x Hope this helps - good luck!
Anthony,First,disconnect the shift cable from the arm on the tranny. Shift into fwd by hand (may need a pair of channel locks) to determine the tranny is actually going into forward gear. If the packing glands are too tight, you'll see the output flange start to try and rotate, but can't overcome the friction that the packing is putting on the prop shaft.Since tightening the packing glands is the only change you have made so far, well, like CJ said, start there...