Can't run through the gears
 

I can shift down into 1st, either moving or at a standstill. When I then pop it over to 2nd, it stays in 1st. I can also pop it over to 3rd, and it still stays in 1st, regardless of RPM. The only way it'll shift is by putting it into drive.

If I leave it in drive, it goes through the gears (starting with 2nd) just like it's supposed to, smooth as silk. What could be keeping it from manually shifting through 1st, 2nd, 3rd and 4th when I feel like rapping it up a little?

this is gonna sound bad, but my car was doin stuff like that before the tranny died. It would stay in first, and then skip 2nd all together. But yours doesn't sound as bad, because it is willing to shift normally if it's in drive. I know this doesn't help, but keep an eye out, cause it could spell trouble.