Ford Explorer; year 1993; mileage 176,000; 6 cly

As I put my car in drive I have to hold down the accelerater down then the truck will slam into gear goes. As the truck gets driven for a while you do not have to do this. But now that the truck has sat for a couple of months when you drive it the problem has only seem to get worse. I was talking to another Explorer owner the other day and he said that it could be that the shifting silinoid could be stuck with stuff from haveing dirty transmission fluid if it hasn't been changed in a while is this a possible fix, because he said it cost a lot less than actually replacing the trany.