main reason is highway cruising .. going 80 seems like its hard on the engine ..about 4000 - 4500 rpm

Much easier to get there with a replacement rear axle.

enlighten me please ... what axle could i get and from what model .. this is plug and play stuff right ? no mods or anything ? how would it change my lower gears performance ?

im satisfied with everything except for highway cruising .. it probably gonna come down to what i want more right ? either low or high

I don't know 126s that well but would think all 126s would interchange, and perhaps some other models from the era. Sixes tend to have shorter gearing, i.e. higher ratios, while V8s are geared taller, i.e. lower ratios. Your axle ratio is on your production data card and on the center differential. You should be able to go at least 20 percent taller, maybe 25 percent. Gas mileage should improve but off the line acceleration may suffer a bit. With the number of 126s in the salvage yard parts should be cheap, $100 - $200, labor maybe 6 or 8 hours, potentially a DIY job. An MB salvage yard should be a wealth of information on what will fit what, or EPC would be helpful too.