Differential Swap?

[Green Dragon]

Hey Dudes. I've been reading this forum for quite some time now researching my future X5. You guys are great! I'm torn and go back and forth on which engine and transmission combo to go with. I've owed 3 BMW's (2 e30 318i's and an e36 325i with 320,000 miles and counting!) I have had bad luck with auto transmissions. After my 325i ate up two autos, I decided to rip the boat anchor out and put in a manual 5-speed. Waaay better. I am not abusive and am very meticulous with maintenance and I do all of my own. I know there are a few that will disagree with my statement but I really think these auto trannys are weak sauce and from what I've gathered, they haven't gotten much better. So I'm leaning toward a 3.0 with a manual 5 speed. The '04 and newer is a tad out of my price range. (Still in school, getting ready for med school.)I've read that they turn some high RPM's at cruising speed. I got to thinking, are the 4.4's (3.64:1 ratio) differential compatible with 3.0's (4.10) differential? If so you could swap both front and rear differentials, you gain lower cruising RPM and perhaps an increase in fuel economy. 71mph vs 63 mph @ 3000 RPM for one example. What do you think? Anyone know? Would this make it too lethargic? Thanks for any input in advance.