Shifts with a jolt sound more like a transmission issue. Maybe the converter is going bad and you need a better transmission shop for diagnosis.

Timing isn't something that changes on newer cars. Unless you have lots and lots of miles and need the valves adjusted. According to the local mechanics around here, the 6VE engine rarely needs to have that 100k mile valve adjustment -- to perform well.

It can't hurt if you want to spend the money, but it's usually not necessary for continued service.

Obviously, I'm not answering your question because I don't live in your area to point you to a mechanic. You only need to find someone familiar with Isuzu's vs the VX. Because it shares mechanics with the Trooper/Rodeo, it's not really a mechanic's mystery.