Pull the rear low/reverse servo out, and stick a screwdriver into the bore where the servo shaft goes to apply the band. You should feel the spring action of the band. Make sure that it's properly aligned with the case. Check the 2 seals on the servo as well as the cover gasket with the 4 holes. Use a 1/4" drive In/LB torque wrench to properly torque the bolts on the valve body. Check the valve body gasket for any flakes, chips, or pieces which could have broken off into the pan. Check the manual lever. It could just need an adjustment. Do your reverse lights turn on when you put it into reverse? Did you see if it's possible to start the vehicle any other gear setting other than park or neutral? You might have a sticky valve/spool in the valve body. A valve body which has too much torque in one part, and not enough in another part could twist it enough to bind the inner parts.