Putting gear-motors in a tube for axle is 'number 1' when it comes to dependability for wheel loaders, articulated dumpers and so on, in my opinion. To get enough power for like 5'' wheels one should go for 540 motor size, that will say the pipe should be like 40-42 mm outside, a standard tube called 1-1/4'' I believe (need to be machined to size internaly). There are different kinds of gearmotors with a 540 size motors around, but many of them are the typical 'china ebay' stuff, too weak. If you could get hold of this type of motor as shown in the link, you don't need to worry about power...The one that have 50-60 rpm at 12V (approx 5'' wheels) They are branded 'MFA' in Europe. This company are located in England, I would think they possible ship overseas too.
http://www.mfacomodrills.com/gearboxes/942d_series.html