1999 Sentra GXE ATX - Shifting Hard & fuel filter

[searcherrr]

Hard shifting - The subject says it all. Shifting hard in 1st and 2nd on all accels from a stop whether the car is warm or not. Car only has 55,000 miles on it and it would be strange for tranny trouble to be occurring already. Recently found that the tranny was 3/4 quart low on fluid. Shop found no leaks, but they were curious about how the fluid went down and no leaks. Ideas? Sensor for gears somewhere? Possible Throttle position sensor problem? Possible "fuzzy logic" chip gone bazerk?

Fuel Filter - Where on the car is it? Anything special I should know before/after I replace it? Is it hard? (IE: I have replaced a starter on a van before and did that pretty easily.)

Tranny oil change - Supposed to drain it; fill it with Tranny oil; flush/drain it again (do I turn the car on after filling the 1st time first before draining?); and then re-fill it again. That right?