Assuming this is the ZF4HP22 box & you say the fluid level is correct.
What's the mileage?
Does it drive forward if you select '1'?
There are a number of tests in the manual but you need the proper pressure gauge to screw into the box.
Q: My Autobox is now only able to select reverse. Why?
A: Your A clutch (forward drive) has failed and you will require a replacement autobox.
www.ashcroft-transmissions.co.uk/frequently-asked-questions/4hp22-faq-s.html
Chances are the friction material on the A-clutch plates has worn away & whilst the A clutch is (relatively!) easy to access, check & replace the plates which are a mix of lined & plain the worn material will be all over the inside of the box, torque converter & cooler as a fine dust & unless completely stripped & cleaned will make it's presence known as it will block the governor & cause shifting problems later.
Which is why a replacement - incl torque converter as the old one will also be full of bits - would be the best idea. Thoroughly flush the cooler as you don't want a lot of muck washed into the replacement!
These links might be of interest:
www.bimmerforums.com/forum/showthread.php?1852917-4HP22-A-pack-information
http://www.cowdery.org.uk/zf.php - (note that the rear housing on the LR is different).
Used these for some parts for mine - mostly gaskets & seals & the teflon rings mentioned in the bimmer link - to cure a few leaks last year. Easy company to deal with:
https://jpat.co.uk/zf.html
Exploded view of the box (again, rear housing on the LR is different)
https://jpat.co.uk/uploads/1/0/3/5/103549764/89__zf_1_4hp22-24.pdf put the pics of the parts (pages 1 & 2) side by side & it will make sense. 'Forward clutch' is your problem.
When removing the gearbox c/w transfer box I'd advise making up the cradle shown in the manuals as it's a big heavy unit.
Be aware the torque converter is only pushed onto the input shaft & will happily slide off & fall on the floor. It too is very heavy & still contains lots of ATF which will go all over the drive. Guess how I know...
I also put a concrete breezeblock on its side under each wheel to raise the car slightly as the first time I took it off many years ago it wouldn't then slide out from under the car as the bellhousing was higher then the available gap
ETA if this link works - sometimes it disappears - it's very good for showing what you're needing to look at. Note that you don't need to remove the box sump or anything at the back of it to access the A-clutch:
http://bmwe32.masscom.net/moswald/zf4hp22_rebuild/zf4hp22_rebuild.html
Can be a fiddle getting the teeth on the plates & covers to line up when you put the basket back onto the gearbox but a right-to-left twisting motion helps. It's fully seated when the gap between a straightedge across the front of the basket & the gearbox front face is no more than 8.5mm. Also helps to have the gearbox vertical - same when fitting the torque converter.
Whilst the TC CAN be fitted with the box horizontal it's an utter bastard - vertical it's a piece of cake! It too must be seated to a specific depth BEFORE offering the box back up to the engine.
