How can I remedy posdible transmisdion slipping?

What is your question?

When I start my veh. and start driving it goes into high gear like 3rpm or 4 rpm whatever that is when its is suppose to be in 2 at a speed of 30mph. It eventually shifts out but very slowly and it does not accelerate correctly unless Im already driving at a high speed. The odometer reads 255530 something but the engine wasvreplaced and only has about 150000 miles on it or less. It does this very often to the point I dont frive on interstate because of the lack of acceleration. Is this something that can be fixed without a hefty mechanic bill. Please help?