as stated
pinion seal is leaking and am losing power above 50mph

i plan on replacing the seal but what would cause the lack of power???

Not enough information to even begin.

What model truck, year, engine and transmission???

Probably your two issues are not related to one another.

bad gas, not enough pressure applied to the skinny pedal, not up shifting into a higher gear, blown headgasket blah blah
like millball said you need to provide info and your two issues are probably not related unless you have lost so much gear oil you are smoking your third

87 4runner
5speed
250,000 miles on it
original everything for the most part in the driveline and rear end

the problem isnt engine mechanical.

pinion seal on rear diff is allowing all kinds of spray
i get that part

question is when i get over about 50mph it just wont go and the rear end tremeors a bit

wondering if bad u joint would cause the loss in acceleration and or shudder in rear.
if not what would the culprit might be

U-joint, driveshaft, and loose pinion faults can all cause shudder and/or vibrations.

Your lack of power problem is likely a separate and different issue.

How long have you had this machine?? Did it ever run stronger than it does now, while under your care?? Did it recently come into your hands??

Have you checked for any stored fault codes??

had it since 2014
not my first toyota.....more like my 18th since 1990
lots of mr2s , taco, and three tundras
third 4runner

never had this issue though
the engine is smooth and fine

the power just isnt getting to the rear.
never had to deal with a rear end before and or ujoints
been reading for about two days and just figured i would ask the veterans here.

Do you do your own mechanic work??

18 Toyotas in over 25 years ought to make you an expert with these machines. Or maybe you are a serial car killer, eh?

Just because an engine will idle and rev smoothly without load, doesn't mean that it will make power and pull.

I'd look to fuel pressure issues, ignition timing, and TPS calibration for some starters. When was it's last tune-up? Fuel filter change? What has your fuel mileage been??