My 740TD left the factory with a ZF, swapped to an M46 by the PO. He left the rear axle in, the ratio is 3.91, factory M46 cars got a 3.55. Swapping in a ZF wouldn't be a picnic, but I have one, along with the brake pedal assembly and swifter, I don't have a cross member for it or the drive shaft. The ZF 4HP22 was used by quite a variety of manufacturers including Jaguar and BMW. Sounds like your M46 has seen better days. I would check the oil level.
