There are a few issues that can cause failure to shift out of 1st. The most common problem I has seen is the front band being either burnt or too loose.
Fluid and filter change is the first step. (after the fluid and filter change drive it around for about 5-10 miles using a method described below to expel and residual air in the system if the oil was low and sucked air)
First thing i'd do is check the color of the trans fluid. If its dark brown or black, i'd suspect the band is burnt. To confirm you would need to drop the valve body or get and inspection cam to see up in there. One of the common problems is someone trying to adjust the transmission band, and accidentally unscrewing the adjustment screw. This will allow the band anchor to fall out of place and onto the valvebody or into the pan. When that happens the shift solenoid has to extend to its full length, which isn't enough to get firm engagement. The result is either no 2nd gear, or 2nd gear that slips back to 1st with the slightest touch on the gas.
Have you tried flooring it all the way to the redline near 5400rpm. That should get you up to about 40mph. When you hit 40 or above (don't worry about bouncing off the redline a few times if its warmed up) put it in neutral until it drops to idle, then put it back in drive and lightly hit the gas. If you have 3rd gear/OD my money's on that front band. If not, it could still be the front band, but it also opens up the possibilities of something else.
Unfortunately, as I recently found out, there is no way to replace the second band unless you have a slip band I think its called, but I cant find any for the 46-48 series of transmissions, without a rebuild. Good news is, you should be able to remove it without much work in way of the rebuild. And you can get a nice red eagle rebuild kit that will take care of any gasket or o-ring problems you might have. Plus it will be all ready to handle the power should you decide to build and tune the engine later on.
