Hello. I have a 1993 K-Blazer (350 TBI 235K) and when I shift into 4HI while moving, it will not engage. If I come to a stop and put the truck in Neutral and rev the engine a bit, it sounds as if there is a gear that engages eventually. It sounds like when you're moving and you put the car into Park too early, like something metal is running over the teeth of a gear until the spinning of the gear slows enough to engage. Then the front drive shaft engages with a loud thud. Also, once engaged, when you have your foot off the gas for a while and you hit the gas again, there is another loud thud as the front drive shaft engages again. I'm not sure whether to start with the vacuum actuator (if it's still vacuum on this year) or the TCCM? Can you tell me which one would be a better place to start? Thanks much!