This is an automatic, right?
You could be just low on fluid. It's pretty easy to check the level yourself (in most cars).
Somewhere under the hood there should be a transmission fluid dipstick. It's basically like checking your oil. I'm almost positive that the last time I needed to check this, the manual for whatever car that was said the engine should be running and the transmission should be warmed up. Check the owners manual for the location and procedure on your Isuzu.
If the level is low, add whatever type of fluid that the owners manual recommends (specific brands need specific types). Your local auto parts store (AutoZone, Napa, Express AutoParts, etc, etc) should have it. That might just take care of your problem. Keep an eye on the level though, you might have a leak.
Look at the color too. Fresh automatic transmission fluid (ATF) is bright red. Almost pink. As it ages, little bits of metal are worn off your transmission and the ATF turns more brown, and eventually black. If the ATF dipstick reveals something that looks like used motor oil, you're in need of a full flush/change.
Most quick-E-lube places can do ATF changes. I want to say it's like $75, but I don't really know. Whether or not you trust them is entirely up to you. I was just at my Chevy/Pontiac dealer the other day for an oil change and I'm pretty sure that it said that ATF changes were like $99. Not too bad. You should be able to call around to different mechanics and/or dealerships for quotes. An ATF change is a pretty standard thing, so they ought to be able to quote you a price.
If it's a manual transmission, your clutch may be on it's way out.
