Most good shops will do a road test and diagnosis free of charge, which is your best bet. If you want to check some things yourself try these. Check all the mounts, including, Motor, Trans, transfercase, and crossmember. See if anything is shifted out of place, of if something is stopping the shifter from moving to the full posision. How many miles are on the vehicle? It does sound like the shift fork inside the trans. is worn out. If you ever see one they are a little piece of alum. and do get worn down. It could also be the schronizers(sp?). My sugestion would take it to a good shop and have them road test it. It is also good to use the same mechanic, or shop, because we get to know the vehicles and owners and that helps out so much. We know what is "normal" for the vehicle and become friends with the owner.
My 2 cents.