03 F-150 fx4 frontdif

Ford f150 fx4 4x4 automatic tran4x4 shifter in the floor40000 miles

front end 4x4 will not engage all the time ----just started doing this----when it does engage it seems to be really stiff in the steering----stiffer than I have noticed in the past4h is all I have had it inno off road useactuator on top of diff seems to be working----when 4x4 is not working drive shaft is still turning as normal and all lights are normal----can you shed some light on this problem
thank you for your time----