A 6-spd dually means he probably towed heavy.
Check for clutch slippage.
Get all service records possible.
MSN pulled their checklist, but here is a similar link.
How to check a used car - illustrated used car checklist
Here is a previous discussion link, but it is for 2nd gen.
http://www.cumminsforum.com/forum/98-5-02-powertrain/333149-new-cummins-what-look.html#post3496310
good luck
Check for clutch slippage.
Get all service records possible.
MSN pulled their checklist, but here is a similar link.
How to check a used car - illustrated used car checklist
Here is a previous discussion link, but it is for 2nd gen.
http://www.cumminsforum.com/forum/98-5-02-powertrain/333149-new-cummins-what-look.html#post3496310
good luck