Proton NMR line widths and relaxation have been studied in powders of almost simple cubic NH4ZnF3 and NH4MnF3. The activation energy for NH4+ reorientation is Ea/kB = 650K above Tc = 115 K in NH4ZnF3 and Ea/kB = 795 K below Tc, and probably about the same in NH4MnF3. The averaging of local fields because of rapid reorientation narrows the line and permits observation of critical relaxation near the antiferromagnetic TN = 75.8 K in NH4MnF3. If T1 ∝ (T - TN)ρ we find ρ in the range 0.1 to 0.3, compared to the expected ρ ≈ 0.33 for cubic Heisenberg antiferromagnets.