1994 Jeep Cherokee Transmission Service

I have a 1994 Grand Cherokee Laredo. It has almost 400,000 miles on it and we have not had any problems until just recently. Our transmission has started to stick in 2nd gear and then hit 3rd. It doesnt do it all of the time though. Only when you dont drive it really slow and let it carefully go into gear. We are kind of nervous about getting a transmission service done and were wondering if we should be. I dont know the history of this jeep but the people that we bought it off of took well care of it. To look and hear this jeep you couldnt tell that it had that many miles. This is the first problems that have arrised out of it. Do you think that it would be safe to do the service and is there a special type of oil that we should use?