What will put a 2006 Jeep liberty 4*4 into limp mode. Not going over 35-40 mph. Making it seem like it isn't being told to shift in the tranny.

It cold be that the trans is only working to start off because of something to do with your overdrive. The computer is literally putting the trans into limp home mode and why you have no drive to get the vehicle moving, yet it will move in second. I've seen this many times, yet you say it will have drive after 15 mph. Really, this makes no sense, it should still be in limp home mode and no drive. If you can get speed up and have overdrive, yet the OD is intermittent or feels like slippage somewhat, then your overdrive clutch assembly, hub and band are burnt. Have it checked by a transmission shop but don't tell them what you were just told by me. Let them figure it out with testing equipment. It's ,ore reliable than the word of mouth thing.

Here's the link and I'm sure you will get the solution of your problem
Jeep Grand Cherokee Service Repair Manual 2005 2006 2007 2008

Well code 755 is for a problem with the 2/4 shift solenoid circuit inside the transmission.
Someone would need to check out the solenoid and the wiring if and when the problem is constant.

I have the same problem on my 2007 Mercedes E350, and cant get an accurate answer from any body, every body tell me different things even Mercedes Dealer. Help me!!!

your car is going into limp mode, check your tranny shift control module, or shift position sensor.

The flashing light indicates you have a big misfire that could damage the catalytic converter.. Being you were going uphill (under high load) this would indicate an ignition issue. You may have a bad coil or wires or a bad spark plug. The reason you could only go 30mph was because the computer was put in limp mode. This is a stratagy to prevent excessive damage to components. , Get it checked out ASAP

scan it.check vechicle speed sensor.a bad speed sensor will cause shift problems.

sounds like to prndl switch may be malfunctioning.
gear selector switch, which tells the transmission which gear it is in.

Ratio codes generally indicate a pressure loss & slipping which the computer reads as a gear ratio problem. Likely you need a rebuild.

Transmission has gone into "limp" mode. Have it scanned soon.