I have done it the past. It's best to pull both together. It's not worth dealing with the hassles of disengaging the torque converter and then pulling the engine.

Once outside, you will have complete access to replace all the seals on the outside of the trans before putting it back in. Most seals/o-rings are are easy to change out.

I have also replaced just the tranny with the engine in the car and getting them aligned was harder than when I had both of them outside. I purchased a transmission jack and that did the trick.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold