could be a motor or tranny mount? i'd get someone to look at the engine while u shift it into gear and then reverse with u'r foot on the brake. also could be a u-joint do the same thing and see it the drive shaft jolts. i aske dthe same thing about how to tell the diff between tranny's someone posted these sites for me to tell the diff between the pans.
http://www.charlietranny.com/FMX.jpg
http://www.charlietranny.com/C6.jpg
http://www.charlietranny.com/C4.jpg
hope they work and it helps