Shifting problems

I have a 1989 Chevy Blazer. I have had the transmission rebuilt twice. It is an automatic w/ overdrive and 4 wheel drive. It just recently started have troubles shifting. When you first take off....it will act like it is neutral until you let it coast for a while or push the gas petal to the floor. Then it will act like it is in first or second gear until you hit about 45 mph...it slams into 3rd gear. After that it drives great. I think it might be a vaccum leak or something to do with the hoses....any suggestions? Where should I start?

Is your SES light on. If so take it to autozone and get it scanned.
Post the codes here. that will help us help you
thanks