Caution: a change to ATF+4 may induce even more "slippage" in the transmission at this point. If it already feels loose on Trick Shift, which has frictional properties not unlike Type F, the transmission could be most of the way to the junkyard. They could have been using the Trick Shift as a band-aid just to get it to shift cleanly enough to sell it. Then again, the transmission could be just fine, but the bands or TV cable are way out of adjustment. On the 44RE, at least one of the bands' adjustment mechansim is accessed by removing the transmission pan, so if the 42RE is the same, you'll want to be prepared for that when you do the fluid and filter change.
Does the loose shifting also appear to be occuring too soon in the RPM range, or does it appear to be shifting at the right time...just too soft? A loose TV cable would produce soft AND early shifts. Tightening up the TV cable will firm the shifts, but they'll also come higher in the RPM range, even with light throttle. Very nice that you can "tune" it to exactly how you like it.
Does the loose shifting also appear to be occuring too soon in the RPM range, or does it appear to be shifting at the right time...just too soft? A loose TV cable would produce soft AND early shifts. Tightening up the TV cable will firm the shifts, but they'll also come higher in the RPM range, even with light throttle. Very nice that you can "tune" it to exactly how you like it.