1998 Ford Ranger RPM

Hello.I have a 98 ford ranger with 3.0 v6 engine, manual transmission. My problem is that when the truck is rolling the rpm's stick at 2000 while. I'm coasting or shifting. If the truck is sitting still, everything is fine, but if the truck is rolling at any time the rpm is stuck at 2000.I unplugged the rear differential speed sensor and everything seemed fine so thinking that the sensor was the problem, I bought a new one, and when I plugged the wire into the new one it went right back to the same problems. So now I unplugged it again, and now the rpm's work fine again. The only problem is the abs light is on while wiring is unplugged from sensor. I also want to add that when I am coasting and rps are sticking, they go down as soon truck comes to stop