4L65 not engaging

[Steve Chryssos]

That sluggish, "not enough fluid" sensation more than likely means it's in limp mode, which means 2nd gear only. There are a million reasons why that could be the case. But there are some easy "external" checks such as:
-- Does the fluid smell burned?
-- Check case harness connections. Literally unplug and reconnect the main case plug and vss plug at tail.
-- Are you surely getting a TPS reading? Grab a multi-meter and check it.
Beyond that, while your waiting for software, you might as well drain the fluid and pull the pan to see if there are any major bits lying around. That will also be a good chance to freshen the filter and possibly the internal harness, if you're feeling spendy.