I had a similar problem with a T56 Magnum and it was the shifter base leaking where the "tower" bolted on. Chased it for over a year. Finally got a new shifter which solved the issue.

I RTV'd the crap out of it. Very bad design with poor quality internal seal on the aftermarket McLeod shifters designed by Red. replaced the seal on the old style shifter 4x and it would tear or harden (poor quality rubber) and fail.