I recently bought a '95 Ram 3500 Van that needs plenty of work, and one such project will be replacing the sliding side door. The door is off the track and will pop out in the back, but the front stays locked in the frame and won't budge. I am wondering if it's possible to completely swap the sliding door for barn doors.

 

Not without some serious body modifications....... I think it would be easier to fix the slider, regardless of how screwed up it is.

 

I have the slider door on my 99 too and for a work rig it was what I had preferred.
Where is it off the track, top or bottom? I had to replace the front latch a handful of years ago, not any to be found in the wrecking yards here but it was still available online in the ball park of $150. Took me a couple years before deciding to finally change it so had pulled the latch access panel and pull on the release rod to open when needed to.
On the rear latch it may be only that a retaining clip came off and the rod popped out of place for you? Never have had a problem with that one other than an adjustment to the catch post

 

It's off the bottom. It seems to be a bolt that the door is caught on, the handle still pulls up and the back pops out.

 

Is the front latch releasing??

 

I could take a couple pics for you in the morning... What i've got is One lower track wheel located near the front of the door and Two above (one near the front and other near the rear of the door. From what I can make out is, that if the lower wheel was off track or the mount for it was loose or broken it might tweak it enough that it might pop out and not latch on the rear post when shutting the door and latching by pushing up on the handle and/or the front will get hung up and not release
You can't see any of that if the front latch won't release as HeyYou mentioned above ,the door has to be open... There's a small panel, maybe 8"x10" with just 4 screws that secures handle where you can access the front latch and connecting rods. Once inside, the long horizontal rod at the top (as I recall) connects the front and rear latch. That should release them with an easy pull on it

 

on the older. "square-r" body vans it was pretty simple task.... I hated the single back door on some, my uncle had one that we swapped for the double doors... same hinges for both options but the single door more than 2x heavier, more stress on those puny hinges..... ive seen the slider swapped for double doors (haven't done one myself though) on those older models. I don't care for the slider myself

 

It wasn't a perfect fix yet, but is functional. I replaced my sliding door, which kept breaking, with the double doors. There is 1 main problem. The step on the sliding door is much lower than the one for the doors. I bled a2x6 to the bottom of the step to mostly fill the space, and I intend to weld a piece of metal in eventually.