Hi there,
I'm looking for a car rental company which will let me pick up a car in Nairobi, and drop it off in Arusha. Is anyone familiar with a reputable company which does this?
Thank you!

It's certainly not offered as standard and essentially Kenya rental vehicles aren't allowed out of the country. You might find someone prepared to do it, but the costs would be huge. In practice, everyone either flies or takes a bus.
Sorry :-(
Richard
take a shuttle from jkia -riverside or impala
This normally depends on how many days you want to rent the car,but if its just for transfer you can use shuttle bus.there are two in a day.Morning leaves at 08:00am and afternoon one leaves at 2:00pm.
For rental there is procedure for you to go through if you have to cross the boarder,the company has to write a letter of authorization,pay for the insurance,this can be obtained from the boarder atmost 30usd,as well as PTA card which goes about 50usd depending with the type of the car.In most cases you will not be allowed to cross the boarder without the concept of the owner.Sometimes back we used Wildlife In Action Safaris and rented their rav4 but it took us sometimes to get the clearance from the boarder.http://www.wildlifeinaction.com,am not sure if they still do the same.
but shuttle would be the best and most convenient.all you need is visa and ticket of course.