Fixing leaks and proper trans service including replacing filter and adjusting bands would almost bullet proof that trans in a stockish truck. Almost every time I have rebuilt one of those (100s of them) they had leaky tranny lines or cork pan gaskets that never got fixed and a lot of times people would come to me because another shop told them it needed a rebuild and I could fix it with simple band adjustments and fresh fluid and filter. Another common problem is the cooler had a checkvalve in it and they would get plugged, just cut that sucker out and bypass it.