Sorry to have to give you bad news but your Astro does not have bands in the transmission. The later model units use clutches and there is nothing about them that is adjustable. On your van the only thing besides the linkage that is adjustable would be the kick down cable that controls passing gear and the up shift. If it is slipping in any gear, the problem will be internal in nature and it will require some specific testing of fluid line pressure and a few other things to find the cause.
Your best bet would be to find a good reputable ASE certified transmission shop and have them give it a road test. Most good shops are more than happy to give a free written estimate along with a preliminary inspection. Just be sure you are comfortable with what they have to say and the explanation of the trouble that they give you. Depending on what year your van is, it may or may not have some electronics that help it work. If you got really lucky there may be a problem with one or more of the shift solenoids. But that is the rare case. And again something that would need o be inspected and diagnosed properly to give you an idea of the problem. If your van is new enough, it may even have the transmission electronics tied into the check or service engine light. If that is the case some of the larger auto parts stores like Auto Zone, are happy to scan the system as a free service for you. Even that may be a good starting point for you. But again it will depend on the year of the van. Let me know what you find out and if you have a question on a specific component just drop me a note. If you do happen to post again, let me know the year, which motor and if it is aXXXXXor something else. and most important any code numbers that may be obtained by a scan. Thanks, bob