I bought this truck a month ago. Drove it 3 times and all of a sudden it revs high and doesn't shift unless I gas it. Won't go past 1-2 gear as far as I can tell. Fluid isn't really reddish and smells a little burnt if you drive it. Thanks in advance.

 

I hate to say it, but your transmission is gone. Probably a 4L60e, but it could be a 4L80e transmission.

 

could be speed sensor that goes into the top of the transmission my truck did the same and I put the one in on the driver side on top of the transmission and my truck started shifting again.