1991 Nissan 300ZX Major tranny problem

I just bought a 91 Nissan 300ZX. It has a problem with shifting out of 2nd. More generally it poses a problem any time I put it in D, but more often when overdrive is disabled. I'm not very knowledgeable when it comes to transmissions, but I was thinking it was something to do with the D gear itself. A mechanic I talked to said it sounds like it might be the linkage? Any body know anything about this? Any help would be greatly appreciated. Thanks in advance.