I'll try this to fix the problem I have. I have a fully raced prepped 60E in my '06 Sierra 1500 and it does the same thing in 4th. As soon as I let off of the throttle it unlocks. Since it has a 3200 stall billet converter in it when I get back in the gas it runs the rpms up then when it re-locks it slams back. Its an annoying PITA.