Okay...I am EXTREMELY familiar with the ZF4HP22...I bought my LWB with a blown tranny (no forward motion, only reverse). What has happened is a pair of steel seals have worn and allowed high pressure into the first gear clutch pack. However, if your fluid is as dark as you say than more than just the first gear clutches are gone. I bought a Master rebuild kit ($230) to make sure that I had everything I was going to need. It is not too bad to rebuild...just take your time and keep everything clean and organized. You will want to check all the gear packs for worn clutches...more than just my 1st gear was worn, and my fluid was only dark red.
Here's a link to check out:
ZF 4HP22 repair / rebuild info
Also, the factory service manual (pre 95) does a good job of breaking it down. If you need the PDF of it PM me.
Good luck, and ask lots of questions.
BD
