rocking carriage?
my first thoughts are....
if this is an older unit alot of them had side thrust adjusters inside the carriage roller stub shafts. The way to tell if it has these is to look on the inside vertical bars of the carriage where the rollers are and if you see a hole with a setscrew and nut then there is your side thrust adjustment. These were notoriously wearing out and causing alot of carriage slop and had to be adjusted regularly.
The way to adjust them is to loosen the nut, get a wrench or allen socket on the adjust screw and tight it up snug and keep an eye on where the carriage is centered, keep it close to center but as soon as you feel it touch the channel stop and tighten up the lock nut. When you get to the opposing side to set the max play adjust it the same way , till it just touches the channel and then back it off 1/4 to 1/2 turn and then lock it down.
There may be 4 of these, one on each roller so do them all the same.

If this is a newer truck then it may have side thrust rollers that should be shimmed and also the carriage rollers must not exceed side play clearance specs. If it does then the rollers will need to be reshimmed.
Keep in mind that the channel may be bent causing this also if there has been any impact to it at any time. If you have issues adjusting the rollers or side thrust adjusters then you may have to check it further for warpage or other damage.

all the other suggestions made are also possibilities as well

good luck