The lowers arms are the same while there uppers have a L&R. I don't recall any problem assembling mine, but I'm sure that there is enough of a difference that they wouldn't, first of all "look" right, nor would the angle of the bushing tube even align with the frame/adjuster eccentric - which is, I believe the only way to tell them apart (its been a couple years since mine went together and I do have terminal OF CRS).

A few more things to look at:
- Is the R&P steering tightly mounted to the frame? Slop in the steering shaft?
- If it is a 2 or more piece steering shaft, solidly mounted support bearing(s)
- Is there a weld fracture somewhere? Front OR rear
- Is that Jag rear end aligned correctly? Springs correct for a 2500# car, i.e. bump steer shoving the car around?
- Loose wheel bearings - rear swing arm bushings or bearings can contribute.