I had the misfortune of having this issue a year after Colt ownership. You'll typically have a VERY stiff gear lever from when moving it from left to right. Usually up and down isn't so bad. The issue is due to the up/down movement of the gear selector shaft becoming stiff.

Essentially what I did was I removed the little shaft that is flared on both ends (holds this part together and allows it to turn providing the up and down motion) by grinding down the smaller end.

This allowed the part to be removed. What I did then was I slowly grinded the outside of the little shaft down so that it was a less tight when fitted back together as this is what was causing the gear lever to be so tight. Mine was not rusty so perhaps it just expanded/contracted due to the heat/cold.

Once you have given the surface a good grind (I recommend using a small dremel tool or similar) you then need to reattach this part. Now the issue is the smaller flared end is now gone so what I did was I used a bolt that fits through the larger end and because the shaft is somewhat tapered (as one flared end is larger than the other) the bolt will hold itself on the inside nice and tight.
You must find a bolt with a head that is big enough yet small enough to do this.

This is my set up. I used a bolt that was the perfect size, a washer, a nylon nut and then a wing nut for security.
I recommend using a bolt with an phillips or flat head end as a socket won't fit down through the shaft.

From this image, you can see the head of the bolt can fit inside the shaft perfectly but it'll not go all the way through as it will hold itself on the inside once the shaft tapered.

The blue T is a representation of the bolt just to give you an idea of what is going on.
This is it assembled. Once this is all put back together, you must not tighten this nut and bolt balls to the walls. It'll pull the shaft and clamp the arm too much. You need to leave 1mm (even less is better) between your nut and the shaft to allow some play. Over-tightening this will just make it stiff all over again. Do not bind them too much. Use a nylon nut so that you can adjust it and then the wing nut to hold it in to place for added safety.

This is the finished product. The shaft has now been grinded slightly so it doesn't sit too tight inside and it is securely bolted back on despite the flare end having to be removed.
I hope this short guide makes sense. It'll be a lot easier to understand once you have your part on the table. This probably isn't the most sophisticated fix around and I am sure it can be improved upon tenfold but it was what worked for me.
The reason I attempted this is because a replacement part is now currently £242 from Mitsubishi which is a lot of money for something that has just got slightly stiff.
I have since bought a replacement shaft but I did this fix over a year ago and it endured perfectly fine.
Gear selector shaft part no.
2960A046 if you wish to buy a new one.
Hopefully it'll help someone out there.
Regards.