1998 Ford F150 4 wheel drive engagement

Hi & thanks in advance for the help. I have owned this truck for 7 years. From the day I bought it the 4-wheel shift was very tough to operate. The last few years I have had a mechanic "lube" the linkage to free up after non use during the summer months just to be able to shift into 4 wheel high. Now the mechanic is saying he lubed it (WD-40) and he can't get it to budge. What causes this and what needs to be done to fix?

Thanks!