Timing Chain

It's unlikely the shake or tremor when putting it in gear has anything to do with the timing belt. More likely the transmission, or maybe the engine mounts of front end components being loose. Hard to say without knowing where the tremor comes from. If you have a timing chain, it should last pretty much the life of the car. If you have a timing belt and it has never been changed on a car with 193K miles, you are incredibly lucky. Get it changed NOW, then every 70K miles or so after. I'm not aware of warning signs of a timing belt going out. I don't know if there are any. If it ever happens, your engine is likely to be toast. Prevention is your best bet.