If the shaft is loose at the base of the shifter and it keeps loosening up, try applying some Loctite on the threads. A Crowsfoot is the best tool to tighten it, but an open end wrench will work. It's a 19mm or 3/4 inch size.
If it's the shifter knob coming loose from the shaft, are you holding the base of the shifter knob with a 17mm wrench and tightening the locknut (a 19mm)up to it.
The last possibility is that the rubber in the isolator (shifter shaft) has separated, which requires replacement of the shaft (seen this happen to several people).
Here's a pic of the stock isolator.
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There are two flat sides at the base of the stock isolator to put the wrench on.