We have used the H-1 pulsed-gradient NMR method to augment our earlier diffusion measurements in n-alkane and polyethylene (PE) melts and blends with measurements in 12 additional melts and in three series of blends at T = 150 degrees C across the full concentration (c) range. Blends were based on M = 33 kDa PE and contained either n-C50 alkane, M = 1.8 kDa PE, or M = 6.7 kDa PE. The results again agree with our theoretical synthesis involving a smooth transition from Rouse to reptational behavior including constraint release and free volume host effects which depend on T, M, and c. Tube dilation is assumed contained in the constraint-release mechanism; contour length fluctuations are not considered to affect center-of-mass diffusion. The generalized constraint-release term not only reproduces the c and M dependences of both D in fully entangled blends but also mimics or supplants entanglement dilution in blends of PE with (unentangled) n-alkanes n-C50 and n-C12. This approach provides a superior alternative in the interpretation of constraint release by shifting from static entanglements spatially diluted by unentangled species to undiluted entanglements whose release rate is accelerated by diffusing diluent. This interpretation is now consistent with fully entangled blends in homologous blends at all M. These results are in semiquantitative agreement with earlier work in polybutadienes.