You need to take it back to the place that 'rebuilt' your transmission and ask questions regarding your shifting problems.
The overheating issue could be related, but there's no way to tell until you get it inspected.
I had the same problem in the past with a 1969 Chevrolet Impala and learned later on that the cooler that you can keep your car running the longer it will last you. Unfortionetly most cars that are put out today put in smaller cooling systems to cut cost resulting shorter engine and transmission life. Take the van back to where it was worked on last and demand it to be fixed right or get your money back and take your buisness to a well known reliable transmission shop. The over heating could be caused by not having your radiator flushed and replace thermastat and replace with fresh prestone coolant and distilled water with equal amount.