I have an 07 supercrew 4x4 with the 4.6L. When I'm on the freeway doing 60-65 at 1800 rpms and I slowly give it gas it downshifts from 4th to 3rd and give a solid thunk feeling. What could this be?

 

Probably the driveshaft yoke splines. pull the driveshaft and put some wheel bearing grease in the splines and see if this doesn't fix it. Was a common issue on the 97-03 trucks and I'm sure it's not different on the 04-08.

 

My drive line isn't loose. Someone told me it could be tranny position sensor.