4x4 Not Working

hi guys i recently got my '96 regular cab 4x4 auto serviced and new tires mounted and now that i have it back the 4x4 wont engage, usually with the gear selector in "D" i'd go about 5mph then push the little 4x4 shifter into h4 and be fine, but now i cant do that, if i do the same procedure now it makes a horrible grinding noise and wont even go into 4x4. then if i and sit at a stand still either in "D" or Neutral the 4x4 shifter will go very smoothly into "H4" but it is still in 2x4 and cant get anywhere off road. can anyone give me some advice, thanks