I am no expert on this, but it sounds like a problem with the locating keys for the first/reverse gear (if I am not mistaken, first and reverse use the same gear on the mainshaft). Either some debris has gotten into the synchronizer (unlikely unless you have other problems) or the locating keys for the synchronizer have become worn or broken.

The synchronizer acts to match the speed of the shaft with the speed of the gear into which you are shifting. When you shift, the lever slides a sleeve over the gear and that sleeve is located on the gear by some spring-loaded keys or "dogs". Sounds to me that something in that process is not lining up correctly. That is why you can shift into first when rolling--the gear and sleeve are moving relative to each other and they find a place where the alignment can occur. You might be able to get into reverse the same way--start rolling backward and see if it goes into gear.

Either way, I think you will have to dig into it to find out the problem. The parts that are likely to be the problem are available and inexpensive. The only question is how comfortable you feel digging into the transmission. I have done it a couple of times and found it fun and not too bad. You can get a lot of advice along the way from this board or other online sources.

I recommend the vclassics article: http://www.vclassics.com/archive/tranreb.htm The only thing I would add is that if you take the thing apart I would replace the locking keys/ "dogs". They are cheap. Good luck!
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Tim--'72 ES, '90 240