When I was running a detroit locker, I had very similar problems. Since that is not the case, I will have to agree with odhinn that your synchros are probably going down the tube, unless your clutch is so worn that it won't grab enough to let you shift smoothly. Are you having any clutch chatter or funny noises when you depress the clutch? If this is not the case, check your fluid levels and maybe change your tranny fluid with some new stuff, either pennzoil synchromesh or the dealer stuff.
If there's a will, usually there's a way, but then you get to that bolt that you still can't get to with two extensions, two universal sockets, and a contorsionist midget trying to help you.
1999 XJ Four door, mostly stock for now, but I am in the process of building a custom plate bumper.