Probably need to get an OBDII reader and pull codes for us to have an idea of what might be wrong. It could be in limp-home mode for something non-trans related, which the codes would help identify.
You can check the transmission fluid via the plugs on the bottom. It has one that you use to drain the pan (allen-key fastener) and one to drain the pan (hex fastener, 10 or 12 mm I think).
The actual checking procedure is to get the trans up to operating temp, and open up the allen plug. If fluid flows out it is over-full. If not fluid flows you, you need to add fluid into the trans until it starts to flow out. There is a 24 mm fill plug on the rear passenger side of the transmission (tailshaft area). It's a pain to get trans fluid in there, you need probably need a hand pump that goes on 1-quart bottles. The total trans capacity is around 15-16 quarts. You need to use Toyota WS or an equivalent fluid (like Valvoline MaxLife, which is widely available) for the transmission - non-WS equivalent fluids won't work.
IMO if this was my rig I'd get it towed home and go from there. Your auto insurance might cover some or all of the tow. The GX should really towed on a roll-back wrecker since it is an AWD vehicle.