The dipstick tube is a two-piece assembly. There's a slip joint with an O-ring approximately where the brace bolts to the bellhousing. The lower part of the tube is welded into the pan.
The theory is that you just take it apart at the slip joint. Problem is that the O-ring gets hard and dynamite won't get the two halves apart. I destroyed the upper half when I did a tranny filter last year. Went to my dealer and they were out of stock on the part -- they had used the last one the day before. Found one at another dealer an hour farther north. Their service manager told me they stock a lot of them because they assume it will get trashed when changing the filter.
He suggested using Vaseline when reassembling. I did that, but I don't know if that's a long-term fix or if it just makes reassembly easier.
The part cost about $10, IIRC.