1998 Ford Explorer Transmission Problems

Just recently I would reverse my Explorer Sport out of the driveway and put it in drive and it will not go into gear. It stays in nuetral. The first few times it happened I pulled the shifter down into low 1-2 gear then back into drive and it was fine. Few other times I would put it back into park then back to drive and it was fine. Even other times it would stay in neutral when I put it in reverse also. This problem has occured randomly for about 8-10 times now in the past 2 weeks. It seems it happens alot on cold starts also. I do get the truck into gear eventually and it runs strong after that, its just more annoying than anything. Fluid is good and no leaks. 144k on it, had it for the past 15k miles with no headaches whatsoever. Any suggestions?