Pull the big drain plug out. There will be a screen in behind it. If the screen has a lot of garbage in it, it's transmission rebuild time. Don't forget to flush the cooler lines and converter.

This is a common problem on the Xm and usually cause by people using atf fluid (dexron 2 red oil) in the transmission, the reverse clutch pack linings will have broken up jamming the reverse pack so it is permanently in reverse, unfortunately you'll need to pull the engine and strip the transmission

Gunter,

Make sure the transmission harness is in good condition with no shorts or breaks. Switch the forward and reverse solenoids to try and locate the problem. If the lift only moves in forward when switched, solenoid is the issue. If it remains the same, either wiring or Forward Reverse switch. Only other condition that would cause this if the reverse clutch packs were locked together. Make sure fluid condition is good and not burnt. Good Luck.

Hey mate, do you remember what your issue was? Were struggling with the same thing