38 series and LPR rims are 3.8". Standard maxx, MT, MTR rims are about 3.2"
If it's a maxx or truggy rim and it says 2.2" then there talking about the width of the rim.
I've never tried putting MT/MTR tires on LPR rims, but I 've heard of it being done. Some people stretch the tire, some people cut the bead of the tire off.
Here's some pics that were posted on the truggyracers forum comparing stretched holeshots to cut bowties.
There was also a how to from a magazine posted. I don't know how hard it is to do or what chance there is of damaging the tires, so it's up to you. Cut tires will be lighter, more responsive, and probably balloon less. LPR's will have a bit less give the MT's in big jumps and very bumpy areas, but that's what shocks are for. I prefer LPR's over MT/MTR's.
If you do go with LPR rims (stretched or cut), you'll need LPR foams. I recommend using something other than proline, like ofna, trinity "bomb one", or my favourite (but most expensive) AKA. Have a look on A-main for monster truck foams.