1997 Ford F150 wont move

i have a 97 f150 4x4 with a 4.6. i bought a salvage transmission from a friend and was told it was a good one. when you put the truck in reverse the motor pulls down 200 rpms like it goes into gear, but wont move. when you shift into neutral the rpms come back up. when you shift into drive the motor pulls back down 200 rpms but will not move. i have put a new filter and fluid in it. the od off light on the shifter is flashing. got any ideas.