Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
Given the way the problem came to pass, with squeaking noise initially giving way to clunking under the floor, I would be most suspicious of a problem with the front axle. We regularly see these trucks have a vacuum control issue (usually vacuum leak or bad freewheel solenoids) that will cause a squeaky brake-like noise in the front end from the axle being partially engaged, and over time it will eventually start thumping when it decides to engage more.
What I would be doing at this point would be to first put the truck into 4WD and seeing if the condition changes (lessens, goes away, changes pitch etc). This would conclusively narrow it down to a front axle issue and then further investigation can be made with regard to the vacuum lines, free wheel solenoid, the actuator on the axle (diaphragm leak), etc.
If however there is zero change in using 4WD then it would be less likely to be related to that front axle. At that point I would want to put the truck in the air and have a helper "drive" it to reproduce the problem for me (in 4WD to simulate the road turning the front wheels) while I listened/watched from beneath to isolate the location of the thump source.