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Hey guys, I am new to this forum, but certainly not new to RC, but I was wondering if any one knows if there is an internal difference, or just different with the spur and pinion?

Functionally they are the same.

Any differences I have seen are due to age differences, stamping on the motor plate, aluminum or plastic outdrive rings, etc.

XX retrofit trannies have some different internals, different buggy/truck ratios.

as bongo said, the transmissions are the same between the buggy and truck, except for the supplied spur gear on the outside.
Both have an internal ratio of 2.18 It is not until the XX series of cars and the xx retrofit transmissions that the buggy and truck have different internal ratios, 2.19 and 2.6 respectively.

Along the way there were some production changes to the original "LRM" transmission, but most of the parts are interchangeable.

That's what I thought, thanks guys. I knew that the XX transmissions had different internal ratios (it's written on the side of the trany). Just wanted to make sure before I picked up some extra transmission parts on Ebay for my JRX2. Also I don't remember what the LRM transmission is? I just finished rebuilding my JRX2 and thought I would take it out for a rip this summer.

LRM, low rotating mass, is just the name for the standard original losi transmission in the jrx- series.

The one difference I can think of that impacts to some degree whether specific parts will work is the 'pro differential'.

The original LRM did not have a pro differential. Meaning the center diff gear rode directly on the diff side in the middle. Thus there is some friction when the diff is working, causing a less free diff action. Note the gear is pushed against the middle under any load.

There was a conversion kit A-3020 with a larger center hole in the diff gear, and an extra bearing to reduce this friction, freeing up the diff action. Just like many other diffs of the day, for example I machined my Associated 12th scale car to add the same feature. (in that case it was removing a hub on the axle, the gear was unchanged)

My Pro SE also came with different colored gears, claimed lower friction. But still not a pro diff. Otherwise interchangeable as far as I know.

Long winded way of saying that there are at least 3 different diff gears for the LRM:
Original A-3006
Pro A-3021 (and available as part of 3020)
Non pro low friction one, included in A-3024. (I think, I only have it from the Pro SE kit)

The pro version required a special thin bearing, A-6904 I believe it is, 1/4" x 3/8" thin according to the parts list. (not to be confused with the bearing right next to it that goes between the 2 diff side parts that all LRMs have)