Transmission Problem in a DII

I have a 2000 Disco with about 41k miles on it. I noticed that the engine speed was up about 1000rpm at the same speed I was at the other day. There was no clunking noises, but you could just feel it slipping. It only seemed to happen in low gears though, as soon as it would shift up it was fine. I was going up a hill, and all of a sudden I had no forward motion. The only gear to work was reverse and it still went into park. I could select all gears, but none of them would work except park and reverse. I did a search and found it could be a variety of things, low fluid, broke linkage, or bad internals. I have already prepared myself for the worst, rebuilding the tranny. What steps should I take at home first before I spend the money on a new transmission? Also, if I check the fluid, what is the proper way to do it? Thanks,

Chris